What is a Galaxy? …the return
The first time I gave a public talk, I spent an hour describing why galaxy classification is fundamentally important to the study of the Universe, the origins of Galaxy Zoo, the amazing response of the volunteers and the diverse results from their collective classifications of a million galaxies near and far. I showed many gorgeous galaxy images, a few dark matter simulations and even a preview of the Hubble image of Hanny’s Voorwerp.
As I finished my talk and the Q&A began, I braced myself for the inevitably interesting and challenging questions (I seem to get a lot of questions about black holes and spacetime).
A brief pause, and then the first question echoed from somewhere in the darkened auditorium: …”What’s a galaxy?”
Oops. Apparently I’d forgotten that little detail at the start of the talk. So I described a typical galaxy (if there is such a thing): a collection of stars, gas, dust, dark matter, all gravitationally bound together. Then I made a joke about scientists forgetting to define their terms, and we moved on to the next raised hand.
Turns out, though, it’s not such an easy question. Even though my casual definition works fine for most galaxies, it’s not at all an agreed-upon standard. We’ve discussed this on the blog before, and even in the short time (astronomically speaking) since Karen wrote that very nice post, more work has been done to find galaxies that push the boundaries and force us to re-think what it really means to be a galaxy.
So, spurred by a very broad interpretation of a question left for us in the comments on the post announcing this hangout, we decided to re-visit the discussion, covering the various properties a galaxy must have, should have, could have, and can’t have. We discussed the smallest galaxies, found by counting and measuring each of their individual stars. We discussed the biggest, brightest galaxies in the universe, living in rich environments and grown fat by eating other galaxies. And everything in between.
Note: when we talk about Segue 1 and 2, I say that these galaxies are unique because they have low mass-to-light ratios. Despite the pause that indicated I was trying to keep from inverting numerator and denominator… that’s exactly what I did. The galaxies have very few stars compared to the amount of dark matter in them, so their mass is high and their light is low, so their mass-to-light ratios are high. Oops (again)!
Jargon Alert: Your IMF Just Messed With My SPS!
I remember going to a lecture as an undergraduate wherein my professor compared what astronomers do to a hypothetical alien crew on a fast-moving ship that can only take one photo of the Earth as it passes by. We can assume they have a special camera that can see through buildings, but otherwise they just get one photo of, say, a major city, and from that they have to try and learn as much as they can about the human race. How hard would it be to discover that our species has two genders? Or that both of them are required to propagate the species, but only one gives birth? Would it be possible to figure out the whole human life cycle? To discover what disease is? To distinguish between genetics and culture (nature and nurture)? Just having one picture is limiting, but with careful study you can learn more than you think.
The professor was drawing an analogy with the Hertzsprung-Russell diagram in particular, which we’ve talked about before on our hangouts: to make it, you record the color and the luminosity of all the stars you can and plot them up against each other, one point per star. The stars group together in interesting ways in particular areas of the diagram, and it turns out that from this diagram alone you can recover an enormous amount about the life cycles of a population of stars (for example, in a star cluster, a neighborhood of our own galaxy, or a nearby galaxy). You can learn even more if you couple the diagram with spectra of stars from different parts of it. Studying stellar populations has helped us understand the fundamentals of what kinds of stars exist, how they are born and die, and how many stars of any given mass are likely to develop in a galaxy in relation to stars of different masses. That last thing is called the Initial Mass Function (IMF for short). Essentially it says that, when stars form in groups, more low-mass stars form than high-mass stars. Put that together with what we know about how much brighter high-mass stars shine and how much faster they die than low-mass stars, and you can start to understand how whole populations of stars in galaxies form and evolve.
And we can apply our studies of nearby galaxies and groups of stars to galaxies we observe much farther away. It’s a good thing, too, since most galaxies are far enough away that our current telescopes can’t resolve individual stars. We just get the sum of the light from all the stars. That combined light is sometimes made up of multiple populations of stars that formed in groups at different times and now all live together in a particular galaxy. Taking that single picture combining the light from billions (often hundreds of billions) of stars and using it to learn about the stars’ masses, ages and histories is an important process, and there are several ways to do it: one way combines models of stellar populations made by forming and evolving many stars in a computer simulation. This is sometimes called Stellar Population Synthesis, or SPS.
On today’s live Hangout, we once again let your questions guide us as we talked about IMFs and what they have to do with SPS and measuring the stellar masses of galaxies. The work that laid the foundations for today’s study of galaxy stellar populations was done in large part by women (Bill mentioned Beatrice Tinsley, for example), which is fitting since today is International Women’s Day. We talked about that too, and about diversity in general in astronomy. Just as you can learn a lot from even one snapshot of a galaxy, you can do a lot with just a bit of mindfulness about being an ally for diversity (Kyle noted on Twitter that World Day for Cultural Diversity is May 21), be it equality for women or for any other minority groups in science, or indeed any field.
Here’s the audio-only version of the Hangout: click to listen to mp3 version.
And the video:
We’ll post about our next hangout soon; in the meantime, keep those questions coming!
Binary Star System in Cetus
80 Cet, a star posted by Zooite and moderator Infinity on Sunday 20th June 2010 for Father’s Day, is in fact locked gravitationally to another star, both orbiting around each other on their common centre of mass. Interestingly, around one in three stars in our galaxy are found in binary or multiple star systems.
I couldn’t glean much information on the stars but with the help of SIMBAD and Peter Clark (@lightbulb500) from the Young Astronomers website, we both agreed that it’s likely to be a Red Giant paired with a White Dwarf star -please point out if we have the classification wrong!
Sources: Wikipedia and Binary Stars Blitzed.
The Hyper-Velocity Stars Project: Serendipity at its Best
Hello everyone and thank you for reading this blog post. You will read how a group of people, non-professional astronomers, from around the world got together to seek a common goal.
My name is Aida Bergés and my Galaxy Zoo name is Lovethetropics. I’m Dominican by birth but live in Puerto Rico, and I hunt Irregular Galaxies, asteroids, Voorwerpjes and now Hyper-Velocity Stars.
The Hyper-Velocity Star Project was due to one of those many accidental discoveries that happen at Galaxy Zoo. I was looking for irregular galaxies for Waveney’s Irregular Project. I saw an intensely blue star and checked it out to see if it was a white dwarf (it amuses me to no end that white dwarfs are all blue). It wasn’t. SIMBAD said it was a Hyper-velocity star. I kept looking for more irregulars, five minutes later saw another very blue star and checked it too – expecting it to be a white dwarf; but it was another HVS. At that point I looked the name up because I had never heard of that type of star. It is a star moving at a vast speed relative to the rest of the galaxy. Possible explanations for this are being flung away from a black hole; being part of a galaxy merger; a binary system being disrupted either by a black hole, the proximity of another star, or one of the pair going supernova . . . and so on. They can travel at about 4000 kilometres per second, and seem to be heading out of our galaxy! All the ones discovered are massive and blue.
I posted the two stars and a brief explanation on the newbies thread. The newbies thread was started by Thomas Jennings, Thomas J or “Tommy” on the forum, on my “zoobirthday” – the day I came to the Galaxy Zoo Forum. A week or two later, Alice Sheppard, Galaxy Zoo moderator and close friend, asked me to post an Object of the Day. I said yes but had no idea what to write. Thomas J reminded me of the hyper-velocity stars I had posted on his thread.
Tommy and I started to look for papers about hyper-velocity stars and he found a powerpoint presentation made by one of the Zookeepers, Jordan Raddick. At the time Jordan created it (2003), there were only three confirmed hyper-velocity stars. All of this dialog was written on the Object of the Day thread, which meant we started to get the attention of and much help help from other zooites. Mark Redgwell – BlackProjects on the Zoo, from the UK – found more papers. When I wrote the Object of the Day we had found 10 hyper-velocity stars, but they didn’t have an SDSS number and I had no idea on how to get them. Tommy again came to the rescue and found their SDSS ObjIDs, and I posted these onto Object of the Day.
It was a hit! We started getting help and more papers, especially from Stellar, 14, child genius from the UK who should be in college already. We found out there were 16 confirmed hyper-velocity stars, most of which had been found by Dr. Warren Brown from the Smithsonian Astrophysical Observatory and Harvard University. He is the outmost authority on HVSs, and you’ll be hearing more about him in a minute . . .
The question came up on the Object of the Day: How can we find more of these mysterious stars? Many Zooites joined the discussion, especially Mukund Vedapudi from India, Jules Wilkinson from the UK, and Dave, Curtis Garrett and Gargleblaster, all from the USA.
We decided we needed our own place to talk, for one thing because we were burying newer Objects of the Day, and for another this was the time of the forum merger which caused a few technical problems. Waveney (Richard Proctor, UK), our “fairy godfather”, offered his test forum, where a handful of zooites play. It’s more private than this one, but nevertheless we kept getting links to papers and articles from Jamartins from Portugal. By now nine or so of us had decided we were a team.
I have an interview for our “She’s an Astronomer” blog project on October 1st, and as it loomed closer and closer I wanted us to have a first post back on the Galaxy Zoo Forum so as to have a link from my interview, and to invite all zooites to join us. Stellar and I worked for 16 hours with some help from Alice and Half65 from IT – both of whom became part of the team, too – we posted our own thread, started a blog and a twitter account, and got ourselves a gmail account so we could spread the word that we are looking for Hyper-Velocity Stars and help from anyone who would like to work with us or advise us.
Jules, Dave, and Mukund Vedapudi have between them found 40 possible hyper-velocity stars, about half of which have a known radial velocity, and which you can examine here. Our new thread has grown to six pages due to the interest of our fellow Zooites; and yesterday I received a lovely personal message from Oswgeo9050 presenting another possible candidate, just from examining the spectra! She’s just joined us, too.
Help is flooding in, some from astronomically unexpected quarters. Karen Masters , who works at Portsmouth University, runs the “She’s an Astronomer and Galaxy Zoo” project, and has been part of Galaxy Zoo’s work on dust in galaxies, red spirals, and outreach generally, just happens to be a former colleague of the very Warren Brown mentioned earlier! It is due to her kindness that we have just got in touch with him – he has heard about our work from Karen and has graciously sent us pointers on how to find more Hyper-Velocity Stars. It looks like it’s going to be tricky: there is only one of these for every 100 million “normal” stars, and there is in any case an error range of 220 kilometres per second (plus or minus) due to our own Solar System’s rotation round our spiral galaxy!
Just to add to this, on Wednesday 9th September, we heard from ZookeeperKevin that he was about to meet Warren Brown and hoped to set up a collaboration. We had a few hours to decide exactly what sort of help from the experts we wanted, and to send him our first e-mail! This was no easy task, not knowing the ins and outs of academic life. We waited on tenterhooks, our hearts pounding . . .
On Thursday evening Kevin got back to Alice. He and Warren Brown had taken a look at some of the candidates. He believed that none of them were hyper-velocity stars for many reasons, one being that the SDSS people have already combed the data. But, apparently, “one object looks interesting”. We don’t yet know what that is, and just have to wait for him to – and this is the good new – join the forum and talk to us more, which apparently he will be doing!
What will happen next? There are probably more stones to unturn. Even if we fail, we still set up an exciting project and discovered many things – as amateurs who decided to work together. In the meantime, please join us to put forward any questions, suggestions, and ideas.
Very special thanks to Zookeepers Jordan, Karen and Kevin for taking such an interest in our work and kindly providing so much information and hope. We also thank Geza Gyuk from Adler planetarium http://www.adlerplanetarium.org for his enthusiastic help.And thanks to our fellow zooites for being serendipities with all of us!