At Zoo headquarters we like to be efficient. That means avoiding redoing work that has already been done by someone else. Particularly if those others have already spent a long time thinking how to do it best. Getting the images for the original Galaxy Zoo (way back in 2007!) was particularly easy. The fabulous Sloan Digital Sky Survey (SDSS) had already done all the work of taking the images, calibrating them, stitching them together, combining images at different wavelengths to make colour images, and optimising their appearance. All we had to do was ask their servers for an image, giving it the required location and size, and voilà, a image ready for adding straight into the Galaxy Zoo collection!
Life was rather more difficult when we added the special ‘Stripe 82’ images from the SDSS. For these, Galaxy Zoo team member Edd needed to do the stitching, combining, optimising, cutting-out and resizing. The details of how he did that are all here. We wanted to be able to compare the Stripe 82 images to the normal SDSS images, so we tried to keep things like the brightness scaling and appearance of colours as similar to the original as possible. Even so, it took us a couple of attempts to come up with a solution we were satisfied with.
With the Hubble data, as with Stripe 82, creating the images for the Zoo isn’t completely straightforward, but again most of the hard work had already been done for us. For the launch of Galaxy Zoo: Hubble, data was taken from several surveys:
- GOODS: The Great Observatories Origins Deep Survey
- GEMS: Galaxy Evolution from Morphology and SEDs
- AEGIS: All-wavelength Extended Groth strip International Survey
We’ve also recently added in COSMOS: Cosmic Evolution Survey images – more about the nitty gritty details of those images in a future post.
The data calibration business was already taken care of by the science teams for each survey. The next steps, finding the galaxies, cutting out images at each available wavelength and combining them into colour images, was handled by Roger Griffith, who already had a system set up to do exactly that. Roger used a nifty piece of software called GALAPAGOS to manage the business of finding, cutting out and measuring the galaxies. The difference that Galaxy Zoo added to Roger’s system was that, like with Stripe 82, we wanted the properties of the colour images to match those from SDSS as closely as feasible, to enable us to compare the results from each of the Galaxy Zoo datasets as fairly as possible.
One particular issue with making colour HST images is that many surveys only produce data at two different wavelengths. Normally, colour images are made by choosing a different wavelength image for each of the three primary colours: red, green and blue. For the HST images we instead use one image for red, another for blue, and then just take the average of the two for green. The primary colours used in your computer display don’t usually match the colour filters that were used in the telescope at all, so the colours you see are only an indication of the true colour. Nevertheless the colours contain a lot of information: galaxies containing only old stars will look red, while those which are actively forming new stars will often be blue. Getting the images looking right, with fairly similar appearance to the SDSS images, required a cycle of testing and exchanging images back and forth, but we came to an agreement fairly quickly.
The HST images in Galaxy Zoo might not look as impressive as some of the press images you’ve seen from Hubble over the past twenty years. That’s because press images are usually picked specifically for their attractive appearance. The images chosen are often of nearby nebulae and galaxies for which HST allows us to see huge amounts of detail. The objects in Galaxy Zoo: Hubble are much more typical of the huge number of galaxies in HST surveys. Although HST can see much more detail than ground-based surveys, its mirror and field-of-view are smaller than most ground-based telescopes, so it can only cover a much smaller area of the sky in a reasonable amount of time. Surveys with the HST therefore focus on faint, distant galaxies, so we end up with images having similar quality to those from SDSS, which is remarkable given how much further away the HST galaxies are compared with those from SDSS.
The similarity between the images of galaxies in the early universe from HST and those relatively nearby from SDSS is actually a big advantage. It means that we can fairly compare the morphologies of galaxies at these two eras in the Universe’s history. That’s what professional astronomers will be doing with your Galaxy Zoo: Hubble classifications over the coming year.
At a pumpkin carving event yesterday, we (a group of people from Yale astro) tried to come up with an appropriate theme for our pumpkin. Naturally, we decided on the Hubble Sequence of galaxy morphology:
And the artists…
After I had started my post about the planning phase of the HST up to the start, I have noticed that the arguments (which I thought to be a small section in there) for building a Space Telescope in the first place took up quite some space, so I have decided to post this for now and delay the rest of the post into a different one (trying to keep the posts from getting too extensive as I promised last time), possibly the next.
At first sight it doesn’t sound like a terribly good idea to put an expensive telescope on an expensive rocket (which doesn’t mean it’s a successful start at all, usually expensive equipment and large quantities of explosives are kept well apart for a reason) and shoot it into orbit where again many things can (and did) go wrong and telescope maintenance is either impossible or at least very expensive. Why not simply build a ground-based telescope which is a lot cheaper but would still have a much bigger mirror (so can collect more light and potentially create sharper images, see below) and would still be much cheaper and easier to maintain? Where a new camera simply needs to be screwed on (a simplification about which many telescope engineers could rightfully complain), rather than having to employ the (again expensive) space shuttle to even get there in the first place and then having to work in unhandy space suits to get the new equipment in (with the risk to notice that a bolt might be too long and the camera doesn’t even fit in); and if it doesn’t work afterwards, you’re screwed and you cannot simply take it down again and fix it? Not even talking about potentially simple problems like cooling the camera chips for which you need liquid nitrogen or helium, which, on earth, you can simply refill with a tank, but which in space becomes a rather more complicated task altogether.
So at first sight, it looks like people need a very good reason to even think about Space Telescopes, an then start developing, maintaining and upgrading one with new cameras. So what are these advantages that made scientists built the HST (and many other Space Telescopes)?
Well, there are 2 main reasons, one of which solves a problem that was at least impossible to overcome back then and one that solves a fundamental problem alltogether:
The first problem: In principle, the angular resolution of a telescope should be given by the wavelength of the observed light and the diameter of the mirror (or lens). This is simple optics that every physics student learns and it says that the bigger a mirror, the better the resolution. But there is one very large problem: This is pure theory for an ideal telescope in a vacuum. As in the old physicist joke: ‘Yes, I’ve got a solution, but it only works for spherical elephants in a vacuum’. But here, talking about telescopes instead of elephants, the problem really IS the atmosphere which changes the situation completely.
The atmosphere has its upsides for breathing, weather (sometimes we could do with a bit less atmosphere here in England 😉 ), airplanes and stuff, but for astronomers, it’s really just ‘in the way’. The air, or better to say the movement of the air, the turbulences of hot and cold air bubbles, bends the starlight on its way into the telescopes (very much like when you’re looking over the tarmac on a very hot summers day everything is flickering). This basically makes stars jump around very fast (Actually, you can see this effect by looking at the stars. Stars ‘flicker’ at night. Objects larger than this effect, e.g. planets, don’t flicker; That’s how you can tell them apart easily). As the apparent position of the stars jump around faster than the eye or a normal camera can detect, it doesn’t really make stars move in long-time exposures, but it leads to ‘blurring’ of the image (If you want to know what a stars picture actually looks like in extremely short exposures, read this article).
This effect is so big that it easily overpowers the resolution improvement due to a bigger mirror size. In fact, to get the maximum resolution of a telescope at sealevel, a telescope with something between 10 and 20 cm diameter is big enough. Any bigger than that does not increase the image resolution due to seeing. Groundbased telescopes can therefore not see details smaller than 0.5-1.0 arcsec in size, even at the best telescope sites (which are usually in very dry places very high up. Dry weather is usually less turbulent and building telescopes at high altitude avoids having to look through large parts of the atmosphere in the first place, so ‘seeing conditions’ are usually better). For comparison, the Hubble Space Telescope has a resolution of only 0.05 arcsec due to it’s position outside the atmosphere and its size of around 2.5 meter.
I show the effect of seeing in the images above. The top one is a picture taken by a 2.2 meter (so similar to HST) telescope in Chile from the Combo-17 survey. The bottom one is the same area of the sky taken by HST (to be precise, it’s a small bit of the Hubble Ultra Deep Field H-UDF, stay tuned for one of my later posts about this survey). You can clearly see that all the objects in the groundbased survey are blurred and some of the faint objects are actually blurred so much, that they cannot even be seen at all above the sky background. In the space based image, the details and features of the galaxies are visible much more clearly. This is the reason why these images were now chosen to be classified in Galaxy Zoo: Hubble. From groundbased telescopes a classification of galaxies at similar distances is simply not possible.
Of course, a big telescope has another obvious advantage: It collects a lot of light, so it enables us to see fainter objects. This is the main reason why telescopes in the past were built bigger and bigger and this trend continues even today.
Also, there are a few things that can be done to improve the image quality, but most of them go beyond the scope of this blog, but if you want to know about it, read e.g. this article and links therein. Basically, either interferometry (where several telescopes far apart are used, unfortunately, this does not produce a full ‘image’ of the object) or movable and distortable mirrors can be used. The latter is called adaptive optics, a very complicated and expensive technique, in which the wavefront of the starlight that has been distorted by the atmosphere can be corrected by a mirror, which is generally speaking distorted exactly in the opposite way to make the wavefront flat again (The mirrors shape has to be adjusted around 200 times per second). A flat wavefront will create a sharp – ‘diffraction limited’ – image, using the complete power of the big mirror used. Adaptive optics was only developed in recent years on several telescopes. The Hooker telescope mentioned in my last blog runs one, for example, and most big telescopes like Gemini, Keck or the VLT run these facilities, too. Although I did call this technique ‘expensive’, it is, of course, a lot cheaper than building a Space Telescope.
In fact this is one of the reasons why the HSTs ‘successor’, the JWST telescope (also one of my later posts) will not be observing at the same optical wavelengths as HST but will rather concentrate on IR wavelengths. Running adaptive optics facilities, groundbased telescopes now produce images of the same quality as the HST, although on a smaller field of view and only close to stars (they need bright-ish stars to compensate the effect of the atmosphere, although laser guide stars will avoid at least the second problem in the future). The 30-40 meter telescopes that are currently planned around the world will show much better resolution again and using very sophisticated adaptive optics might have a similar or even bigger field of view as HST currently has. In the image below (please click for full resolution), you can see an estimated example for a telecope called OWL (Overwhelmingly Large telescope, not kidding, the image above shows a simulation. The speck in the foreground is a car for size comparison). This was a planned telescope which has now been cut down to 42 meters, we now call it the E-ELT (the Extremely Large Telescope, yes, astronomers are not very creative when it comes to telescope names). It’s design is different (but OWL of course is more impressive and their website provided the images I was looking for), but very recently, its cite has been selected to be on a neighbouring mountain to Cerro Paranal, the cite of the VLT.
As you can see on the left, this telescope would, with perfect adaptive optics (diffraction limited) indeed produce much sharper images with much higher resolution then even HST can provide today. So in principle, the problem of the atmosphere can be overcome using clever techniques. The field of view on which this works today is still a lot smaller than what is covered by HST survey cameras, but this is a matter of technique and might be overcome in the future (if interested, google ‘multiconjugate adaptive optics’). In the past, when HST was planned, non of this existed, so a Space Telescope indeed sounded like a good idea.
But groundbased telescopes have an even more fundamental problem which is simply put impossible to overcome. At ground level, only certain wavelengths in the electromagnetic spektrum can be observed. (Far) Infrared, ultraviolet and gamma light cannot be observed at all from groundbased telescopes as the light is strongly absorbed by the atmosphere. The image on the right shows the height above ground at which light is basically absorbed by the atmosphere. As you can see, only optical and radio (and a bit near infrared) observatories make sense on earth, even on the highest mountains. For any other wavelength, you need a space telescope to be able to observe galaxies at all.
Different wavelengths can be very interesting, a galaxy looks completely different in optical than in X-ray or Radio and all these wavelengths show different physical parameters, e.g. star formation rates (X-ray) or the amount of ‘dust’ in the galaxy (in IR). A good and famous example for this is NGC 1512, a barred spiral galaxy whose center has been observed at different wavelengths by the same telescope. You can see the results on the right. Generally speaking, very blue light (shown in purple) shows very young stars, redder light (up to orange) older stars, red shows dust. Remember, these are all still more or less optical wavelengths, in X-ray or far infrared, this galaxy would look even more different.
The HST works at optical wavelengths, so this was not really a reason to build it, most things (although HST does have some IR filters) could indeed be observed by groundbased telescopes. But as I mentioned above, the HSTs successor, the JWST will exclusively be working in IR for exactly this reason. Optical cameras are not really needed in space anymore (although it’s of course a shame that we won’t have an optical space telescope in the future), but for IR observations it’s vital to be in space. Other famous Space Telescopes in other wavelengths include Spitzer (IR), Chandra (X-ray), GALEX (UV) and XMM-Newton (Xray), all of which are impossible to be replaced by earth-bound instruments.
There are also quite a few focused satellites for certain experiments in space, e.g. WMAP (to observe the afterglow of the Big Bang), Keppler (to detect exo-planets, planets around other stars than the sun) and others, but these are focused projects and not open telescopes everybody can apply to, which of course everyone can at the HST. I will talk about this and some big projects that successfully got their time on the HST in my future posts.
All in all, you can see, there are several good reasons to build a Space Telescope, scientists don’t do this because it’s ‘fun’ or ‘cool’. For observations in certain wavelengths it is still important today, for others it has at least been important in the past. Which brings us back on track: At some point, the decision was made to build the HST (although it wasn’t named Hubble then) and the planning began. But this will be my next post, so stay tuned. I am travelling a lot in the next month, so I might not be able to hold the 2-week schedule, but I’ll do my best.
Previous history of this series:
Who is Edwin Hubble, the guy who gave the Hubble Space Telescope its name? Who is the mysterious guy behind the telescope?
Well, actually, Edwin Powell Hubble is not the ‘man behind the telescope’ at all. He was born on 20th of November 1889 in the US and studied Physics and Astronomy in Chicago. He then, interestingly, went to Oxford, UK (now, of course, one of the main departments participating in Galaxy Zoo), to study Jurisprudence, later Spanish. Given that he was also very sporty (he won several state track competitions and set the state’s high school high jump record in Illinois), I think it is fair to call Hubble a person with multiple talents. In England, he also picked up some English habits and his dress code, some to the annoyance of his american colleagues in later years. I don’t know many pictures of him, the one on the right is possibly the most famous (usually used in scientific talks at least). Smoking his pipe on his desk, he really looks like an English gentlemen of his time (Well, maybe he’s lacking a hat).
Edwin Hubble died on September 28th 1953 in California (his house is now a National Historic Landmark at this location), long before the real planning for the HST had begun. Earlier ideas did exist, since 1923, after it was explained how a telescope could be propelled into Earth orbit and in 1946, Lyman Spitzer (who interestingly enough has his own space telescope named after himself now) had already discussed the advantages (which I will discuss in the next post about the planning of the HST) of an extraterrestrial observatory, but it took until 1962 for the US NAS (not NASA!) to recommend the development of a space telescope for other purposes than observing the sun (two orbiting solar telescopes were in fact already active at that time). In 1965, 12 years after Hubbles death, Spitzer was appointed head of the committee to define the scientific objectives for this new telescope, so really, he is the ‘man behind the Hubble Space Telescope’.
So why is the telescope named after Edwin Hubble then?
After some years of teaching at the university back in the US and after serving in WWI as a major, he returned to the Yerkes Observatory at the University of Chicago, where he finished his Ph.D. in 1917. The topic of his thesis was ‘Photographic Investigations of Faint Nebulae‘ (it only consists of 17 pages, a fact that possibly makes every PhD student cry nowadays). At that time, these nebulae were still considered to be part of the Milky Way, something that was waiting for a real genius and careful observer to be revealed as a mistake.
In 1919, Hubble took on a staff position in California at the Mount Wilson Observatory near Pasadena where he stayed until his death in 1953. Just 2 years previously, a new telescope had been finished at the site, the Hooker telescope (the slightly unfortunate name comes from John D. Hooker who funded the project), a 100-inch Reflector telescope, which today is still there and, after some recent upgrades and modifications (although preserving the historical origin wherever possible), is again used for scientific purposes. With its ‘adaptive optics’ system (see next post) its resolution today is 0.05 arcsec, the same as resolution of the HST. From 1917 to 1948, the Hooker telescope was the world’s largest telescope.
Hubble used this new, state-of-the-art telescope to continue the work on nebulae that he had started in Chicago by identifying Cepheid variable stars in them. Cepheids have the very convenient characteristic, that the period of their variability is a simple function of their brightness. So by measuring their period, astronomers can immediately tell how bright these objects are in a standard system. Measuring their apparent brightness allows to measure their actual distance. By doing this, Hubble noticed that they are far too distant to be part of our own galaxy, but instead are extragalactic systems, islands of stars (and possibly life) in the vast nothingness of space. Other distant ‘Milkyways’, just like our own.
We now call them ‘galaxies’.
Being some of the closest galaxies to our own, most of the objects that he worked on are now very famous, some also through images by the HST. The most famous of all is possibly M31, the closest big galaxy to our own, the Andromeda galaxy, or what Hubble called it, the Andromeda Nebula.
Additionally to his distance measures of 46 galaxies Hubble further took measurements from Vesto Slipher of their escape velocity. This is basically the speed with which ‘the galaxies move away from Earth’ (what we now understand to be the cosmological redshift) and can be relatively easily measured by looking at the galaxies’ spectra, in which all spectral lines, previously known from lab experiments, are shifted by the same amount. When Hubble plotted the escape velocity of galaxies over their distance (we call this a Hubble diagram), he noticed something interesting:
The further galaxies are away from our position, the faster they move away.
This was a pretty radical idea as it proved that the Universe is not a static place at all as was widely believed before. For example, Einstein had introduced an additional term into his cosmological formula in general relativity to make his universe static/non-dynamical (something Einstein called the biggest blunder of his life after he had seen Hubble’s data. Funnily enough, this constant is now back in there to explain the accelerated expansion of the universe. It resembles the ‘dark energy’). Instead, this effect means that either the Earth is in a very special spot of the Universe where everything is flying away from it (a thought that many people, amongst them Einstein, considered wrong. The hypothesis that there is nothing special about the place where the Earth is other than that it is where we happen to live, is one of the basic fundamentals of cosmology) or there was a time in the past when everything was at the same point, much like in an explosion. Of course, we now know it was not an explosion in the traditional sense, but the beginning of time, the Big Bang.
Of course, as with most big new discoveries, these new findings were heavily discussed, not many people believed in them in the beginning. One after another, people started believing in Hubbles results, though, and the view that astronomers have on the universe changed completely. The Big Bang Theory (besides being a brilliant TV series) is now the generally accepted picture today.
As a small anecdote on the side: Due to errors in his distance measurements, Hubble measured the expansion parameter (the Hubble constant) to be 500 km/s/Mpc, which for today’s measurements is a pretty bad value, actually. After new, better data and improved data analysis were used, there were 2 big groups of people debating the real value, some said it was 50 km/s/Mpc, some others said it was 100 km/s/Mpc. For the past 10-15 years, this battle seems solved Solomonically, the value is now assumed to be just inbetween these values, somewhere between 70 and 75 km/s/Mpc. So, although Hubble was very wrong in the number that came out of his measurements, he somehow got the principle spot on.
Using the images that he had taken for his work, Hubble also came up with a system to classify these nebulae and galaxies depending on their appearance. This is what we call the Hubble sequence or the tuning fork of galaxies, and Galaxy Zoo initially used a system that was based on this diagram for their classifications.
As the Hubble Space Telescope was primarily constructed and built to observe distant galaxies (besides of course looking at objects in the solar system and interesting regions in our own galaxy), it was named after Edwin Hubble in honour of his groundbreaking work in this field.
Edwin Hubble has not only got a Space Telescope with his name, but several laws, constants and numbers are named after him, too.
- The Hubble constant as explained above, called H0
- The Hubble time is 1/H0 and gives the approximate age of the universe. It is currently estimated to be around 13.8 billion years.
- The Hubble length is c/H0and is equivalent to 13.8 billion lightyears. This is not the ‘size’ of the universe, but is an important length in cosmology
- The Hubble diagram as described above
- The Hubble sequence of galaxies.
- Hubble’s law
Additionally, there are:
- An asteroid: 2069 Hubble
- The crater Hubble on the Moon.
- Edwin P. Hubble Planetarium, located in a High School in Brooklyn, NY.
- Edwin Hubble Highway, the stretch of Interstate 44 passing through his birthplace of Marshfield, Missouri
- The Edwin P. Hubble Medal of Initiative is awarded annually by the city of Marshfield, Missouri – Hubble’s birthplace
- Hubble Middle School in Wheaton, Illinois—renamed for Edwin Hubble in 1992.
- 2008 “American Scientists” US stamp series, $0.41
(I think when they make you a stamp and you’ve got your own highway, you’ve really made it!)
I think that’s more or less all that I can come up with about Eddi, the post is actually quite a bit longer than I thought it would be, I’ll try to keep it shorter in the future, scout’s honour. For now, I will end with a quote from Edwin Hubble:
“Equipped with his five senses, man explores the universe around him and calls the adventure science”
With this, keep together your senses, especially seeing (the galaxies in Galaxy Zoo) and feeling (your mouse button with your index finger) and help us to do more adventurous science with the classified galaxies that you help us with (hearing, smelling and tasting are only of second order importance in astronomy, unless of course you listen to some music and have a snack while classifying 😉 ).
Thanks and Cheers,
Before I start with a new series of posts, please let me introduce myself.
My name is Boris Häußler (look at my horribly out-of-date website here). I am German but currently working as a research fellow in Nottingham, UK, where I have just recently started my second postdoc with Steven Bamford, whom many people here may know. I have spent the last years (actually, my whole scientific life so far) working on Hubble Space Telescope (HST) data, mainly on the GEMS and STAGES surveys, and have gathered particular experience in the field of galaxy profile fitting, trying to measure sizes, shapes, etc. of distant galaxies. Whereas my previous projects have mainly been working on galaxies at redshift z~0.7, my new job is trying to do similar and more advanced things on more local galaxies, mainly SDSS galaxies, which of course everyone familiar with Galaxy Zoo will know as these are the galaxies classified in both Galaxy Zoo and Galaxy Zoo 2. Initially, one would think that this is a much easier job to do, but as this data is from ground-based telescopes, it proves to be challenging.
This brings me to an interesting position. Although Galaxy Zoo is not my primary science project, I am now connected to the survey through Steven, our galaxy sample and (for now) more directly through this blog. Having worked on HST galaxies for ages, it is of course very interesting for me to see these galaxies now being classified in Galaxy Zoo: Hubble. Having created some of the colour images that both GEMS and STAGES have used for outreach purposes, I have looked at thousands of these galaxies myself and know how stunningly beautiful they can be. I very often got lost on our images, simply browsing around and being fasctinated by the variety of the galaxies. At least in GEMS I know many galaxies by heart and could possibly directly point you to at least some of the brighter and/or more interesting galaxies.
Being kind of an HST expert, Steven has asked if I would want to write a series of posts about HST, an offer that I found hard to turn down, so I’ve decided to write quite a long series about the HST, its history, its future and especially introducing some of the bigger HST surveys, some of which of course build the content of Galaxy Zoo: Hubble now. But before I write and post all this, I would be interested to know what people would actually want to know about Hubble and everything connected with it. So if you have any comments, any wishes, any questions, please post them below and I will try to answer them in the future.
My current plan for the next months contains the following posts, roughly running through the history of Hubble in chronological order:
- Who is Edwin Hubble, the man that gave HST it’s name?
- History of Hubble, the planning and the start 20 years ago
- HST gets spectacles, first service mission
- HDF, the Hubble Deep Field, the first famous survey,
- Another service mission, putting new cameras (e.g. ACS) on HST
- GOODS, the Great Observatories Origins Deep Survey
- GEMS, Galaxy Evolution from Morphologies and SED
- AEGIS , the Deep Extragalactic Evolutionary survey
- HUDF, the Hubble Ultra Deep Survey, the deepest survey ever made
- STAGES, Space Telescope Abell901/902 Galaxy Evolution Survey
- COSMOS, the Cosmic Evolution survey
- The service mission to put in another camera (WFC3)
- Upcoming surveys: CANDELS
- The Future of HST
- HST’s successor, the James Webb Space Telescope (JWST)
If you want to know about anything else, please let me know below.
Thanks and Cheers for now,
This object has the imaginative name SDSS J142005.59+530036.7. It lurks in the Bootes constellation and although it looks like a star, it’s actually a Quasar 15.3 billion light years away from earth going by its redshift. I have a love for Quasars, so I couldn’t resist this one in Budgieye’s OOTD posted on the 6th of July!
In the heart of this galaxy lies a super massive black hole like most other galaxies. This particular one is an AGN, an Active Galactic Nucleus. AGN are super massive black holes in the centres of galaxies that are pulling in material from around them such as stars and gas. This material gets pulled into a ring doughnut shaped accretion disk around the black hole, and as this material swirls round it causes friction, releasing radiation out into the galaxy. The centres of these galaxies can be so energetic that they can outshine the galaxy itself; hence all you can see in the picture above is a star-like object- the nucleus of the galaxy.
This energy can also be concentrated into jets of high energy plasma racing out at near to the speed of light for thousands of light years from the poles of the black hole, and depending on how these jets are positioned in relation to us the galaxy the AGN is lurking in can be called radio galaxies, Blazars, Seyferts and so on. In this case it’s a Quasar, so the jet is positioned so that it’s not quite beaming directly at us. Here’s a great OOTD by Fluffyporcupine on AGN!
And thanks to Alice for helping me out! 🙂
This week’s OOTW features this object (below) from Tsering’s OotD posted on the 26th of June.
As Tsering showed, this seemingly uninteresting blob on the SDSS turns into this in Hubble Zoo:
This is AHZ30000yv, a wonderful collisional ring galaxy! I love seeing the huge differences between the SDSS and Hubble images, the reason why Hubble can see more is because it’s out of the way of the Earth’s atmosphere, so even though Hubble is actually smaller than the the Sloan telescope (Hubble’s mirror is 2.4 meters and the Sloan telescope’s mirror is 2.5) it can see further, taking us visually back to when the universe was around half its current age and making me very happy indeed!
This ring galaxy has a Z (redshift) of 1.432, so we’re seeing it as it was 9.15 billion years ago, just under 5 billion years after the big bang! So how did this galaxy end up as a collisional ring? The ring formed after another smaller galaxy punched through the centre of the galaxy, creating masses of hot young blue stars in the process through all the gravitational disruption.
And I have to quote this lovely post by Budgieye from the comments on Tsering’s OotD 😀 :
It is fun looking at the difference.
There must be lots of UV light coming from it, otherwise nothing would be visible at all on SDSS. At that distance, the ordinary blue light from the stars would be redshifted off the limits of the SDSS detector for far red light.
A nice addition to
Colours of Galaxies in SDSS : Redshift chart
Last time I talked about the Great Debate of 1920, and about Edwin Hubble’s discovery that Galaxies lie beyond the Milky Way. The 1920s changed over view of the Universe – they made it much larger! This time I’m going to quickly outline the basic types of galaxies and the kind of sizes and distances we are dealing with.
Galaxies are usually grouped by their appearance. You may be familiar with spiral galaxies, for example. In fact there are two types of spiral galaxy: those with bars through their middle, and those without. You also have elliptical galaxies, which are basically big blobs of stars. Finally there are irregular galaxies, i.e. galaxies that don’t seem to be one shape or another really. There are examples of each of these types shown below – taken from the Galaxy Zoo data, of course!
The different shapes of galaxies tell us something about their properties, and we’ll deal with each type of galaxy in the next few blog posts. For now I thought I’d end with another of Hubble’s ideas. When he saw these different types of galaxies he tried to understand the different shapes as an overall evolution. He thought that elliptical galaxies might evolve into spirals as time went by. The Hubble ‘tuning fork’ diagram is shown below.
Hubble called the elliptical galaxies ‘early’ galaxies and the spirals ‘late’ galaxies. Galaxies do not move left across the diagram as they evolve, but still the diagram is a nice way to visualise the varying shapes of galaxies relative to one another. Understanding the shapes – or morphologies – of galaxies are a huge part of the motivation behind the Galaxy Zoo project. you can learn more about it on our science pages.
[UPDATE: This post has been modified from its original form to correct some errors on my part.]
This is the first in a new series of blog posts under the title of ‘Galaxies 101’. These posts aim to explore the history and basics of the science of galaxies. I’ll be covering some of people who helped us understand these ‘Island Universes’ as well as some of the basics that would be taught during a first year undergraduate galaxies course at university.
It is fortunate that these posts are beginning in the week of the 90th anniversary of The Great Debate which occurred on April 26th, 1920. The Great Debate – or the Shapely-Curtis Debate – took place at the Smithsonian Museum of Natural History between two eminent astronomers, Harlow Shapley and Heber Curtis. Shapely was arguing that the ‘spiral nebulae’, that were observed at the time, were within our own Galaxy – and that our Galaxy was the Universe. He also argued that the Sun was not at its centre. Conversely, Curtis argued that the Sun was at the centre of our Galaxy but that the ‘spiral nebulae’ were not inside our Galaxy at all. He suggested instead that the Universe was much larger than our Galaxy and that these nebulae were in fact other, ‘island’ universes.
Below is a drawing of the ‘spiral nebula’ M51. This is an observation by Lord Rosse, drawn in 1845 using the 72-inch Birr Telescope at Armagh Observatory in the UK.
With 90 years of hindsight we can now say that Shapely and Curtis were both right and wrong. The Sun is not at the centre of the Galaxy and the Galaxy is only one of hundreds of billions of galaxies in the Universe. But how was the argument resolved? The answer, in part, comes from a very famous name in astronomy: Hubble.
Less a decade after the Great Debate took place, Edwin Hubble used the largest telescope in the world – the 100-inch Hooker Telescope on Mount Wilson – to observe Cepheid variable stars in the Andromeda Nebula/Galaxy. Cepheid variables are a type of pulsating stars whose pulsation periods are precisely proportional to their luminosities. This makes Cepheid variable stars a ‘standard candle’ – an object where the brightness is a known quantity. If you can observe the apparent brightness of a standard candle, then you can determine its distance by a simple inverse square law. Since Cepheid variable stars have pulse rates proportional to their luminosity, if you can measure the pulse rate of a Cepheid variable anywhere in the Universe, then you can determine how far away it is. This is what Edwin Hubble did in 1925 and he calculated the distance to Andromeda as 1.5 million light years.
At the time, Shapely thought that our Galaxy was around 300,000 light years across and Curtis believed it was around 30,000 light years. Hubble’s measurement placed Andromeda well outside our galaxy and showed that Curtis was correct in thinking that the ‘spiral nebulae’ could indeed be other galaxies. Today we think the Milky Way is about 100,000 light years across and that Andromeda is 2.5 million light years away.
The discoveries of the 1920s started a whole new adventure for astronomy. The Universe had gotten a lot bigger and was about to expand much, much more. It is important to remember that Shapely, although wrong about the nature of the nebulae, did correctly assert that the Sun was not at the centre of the Galaxy. This is the kind of Copernican shift that makes people think about things differently and it is important to realise that the issues discussed during the Great Debate were complex. For our benefit though, the Great Debate is a starting point for exploring the relatively new study of galaxies. Humanity’s view of the Universe, and our place within it, has changed an awful lot since 1920. The study of galaxies has had a lot to do with that.
[Andromeda image credit: Robert Gendler]
The Galaxy Zoo project has evolved once again – now we are classifying galaxies from the incredible Hubble Space Telescope! Galaxy Zoo: Hubble is the new incarnation of the Galaxy Zoo project and it continues to allow you to help astronomers with real scientific research by asking you to to visually classify galaxies online.
The original Galaxy Zoo and Galaxy Zoo 2 both used data from the Sloan Digital Sky Survey and recently, after reaching 60,000,000 classifications those projects began to wind down. The timing is excellent though and it allows Galaxy Zoo: Hubble to launch today, for the 20th anniversary of the space telescope. Images of galaxies taken using the legendary space telescope are there for everyone to classify and I recommend that you go and do just that.
A lot of the fainter galaxies look like those seen in the Sloan catalogue from Galaxy Zoo 2 (this is a good sign, since we don’t want galaxies to change wildly depending on what telescopes we use!) however in amongst these there are some real gems to be discovered in the Galaxy zoo: Hubble data. I just found a couple in my first tentative classifications, I’ve shown them here to whet your appetites.
Hubble has now been in orbit for 20 years. In that time it has helped us to understand the age of the Universe, to see more distant galaxies than ever before, to detect the presence of black holes in the centre of galaxies, to witness a comet collide with Jupiter, and much more!
Hubble has captured the imagination of people all around the world and it has given us some of the most iconic images of space that exists in the public consciousness. The image at the top of this post was taken during Servicing Mission 4, just after the Space Shuttle Atlantis captured Hubble with its robotic arm in May 2009, beginning the mission to upgrade and repair the telescope. Thanks to that upgrade, Hubble will continue to provide amazing images and science for many years.
So that’s enough chat from me – go and classify some Hubble galaxies!