Tag Archive | OOTW

A Train-wreck in Pisces

This weeks OOTW features Lightbulb500’s OOTD posted on the 22nd of October 2010.

SDSS J233604.04+000447.1

SDSS J233604.04+000447.1

1 billion light years away a cosmic train-wreck of two merging galaxies is taking place, throwing stars out in streamers stretching out for thousands of light years in a flurry of gravitational disruption. Deep within this train-wreck at least one super massive black hole has awakened as a result of the infalling matter from the merger, making it a Seyfert 1 galaxy according to SIMBAD.

A seyfert 1 galaxy is a galaxy host to an AGN (Active Galactic Nucleus). The matter falling into the black hole forms an accretion disk which in turn creates massive jets of plasma as the disk emits huge amounts of radiation as a result of friction. This radiation gets concentrated into jets of plasma that beam out for thousands of light years!

The angle of the jets mostly determines what type an AGN is. There are several types such as the Blazars, the Seyfert 1’s, seyfert 2’s and so on. This galaxy is a seyfert one so the jet is beaming out at around 30 degrees so that it’s not quite facing us directly. A blazers jet would be facing directly at us, and the jets of a seyfert two would be at pointing 90 degrees away from us.

Eric F Diaz commented in the OOTD thread and asked if the galaxy is a polar ring or not, and Lightbulb500 set up a poll to see what everyone thought.

A polar ring galaxy is where a galaxy punches through the centre of another and as a result this creates a ring. This ring sorrounds and orbits the galaxy at its poles, creating wonderful images like this:

Mrk 1477

Mrk 1477

The Smiling Lens

This weeks OOTW features Jean Tate’s OOTD posted on the 20th of October 2010.

SDSS J103843.08+484916.1

This is SDSS J103843.08+484916.1. It’s a gravitational lens in Ursa Major that Jean calls the ‘Smilie’; and you can see why!

So what is the cause of the arcs around the central galaxies?

Gravitational lensing is due to the curvature of space. Think of a bowling ball on a trampoline, the trampoline is the fabric of space and the bowling ball is the mass – say a cluster of galaxies – bending it. The more mass the bowling ball has the more it will bend the trampoline, and the same goes for objects in space.

The arcs are in fact distorted images of a galaxy that is lurking behind the central galaxies you can see in the image; you can see it much clearer in this Hubble images here. The light from the galaxy behind has followed the curvature of space caused by the huge mass of the central objects, making the light bend around the galaxies as arcs.

The left golden fuzzy has a redshift, Z, 0f 0.426, making it around 4.5 billion light years distant. The galaxy that forms the arcs however could be much further away.

A Cataclysmic Delight

This week’s OOTW features Jean Tate’s OOTD posted on the 6th of October

CV star

This is SDSS J120231.00+450349.1; a Cataclysmic Variable star in the constellation Ursa Major.

Cataclysmic Variables are stars in a binary system, with one white dwarf and another star of varying type. The white dwarf steals matter from its companion as it orbits closely, often completing an orbit within hours! As the white dwarf pulls the matter off its companion it surrounds itself with an accretion disk mostly made of hydrogen. If this CV was observed in the X-ray or UV you’d see it as strong sources in both wavelengths, as both X-rays and UV are being strongly emitted from the accretion disk!

As the name suggests this CV varies in brightness, getting brighter for a period as the accretion disk falls onto the white dwarf, setting off nuclear fusion at the stars surface.

Jean Tate found this CV to be of ZZ Ceti type, which are stars that pulsate, swelling from one size to another. Jean Tate writes:

In the H-R diagram, there is a thing called the instability strip; stars which fall in this strip pulsate (move in and out, usually radially) … and that pulsation is used, in Cepheids, as a key ‘standard candle’ in the cosmic distance ladder. Some white dwarfs pulsate; some which pulsate are called ZZ Ceti stars, after the variable ZZ Ceti: they are hydrogen WDs (classified as DA), and because they are variable, DAV stars (helium WDs (DB) can be variable too; they are DBVs. I don’t know if carbon (DQ) or metal (DZ) white dwarfs can pulsate).

I highly recommend reading her OOTD for a lot more information; and for details on the spectra!

Sombrero and the Ultra-Compact Dwarfs

This week’s OOTW features Jean Tate’s OOTD posted on the 28th of September.


This is M104, otherwise known as the Sombrero galaxy. Lurking in the picture above hanging in front of the galaxy’s halo and blending in with all the stars of our galaxy is a strange little thing; an Ultra-Compact Dwarf (UCD) called SUCD1 :

SDSS UCD Sombrero

UCDs are very compact objects, with millions of stars crowded into a small area as small as 200 light years across! They are rather luminous, showing up on the SDSS as star-like points. The UCDs have been observed in the Fornax cluster, which you can read about in the papers linked to in Jean Tate’s OOTD.

These objects are currently the subject of a lot of debate; are they dwarf galaxies or aren’t they?

Jean Tate summarises this, calling into question if the dwarfs are galaxies or more like globular clusters:

The jury is still well and truly out; however, UCDs do fit several (elliptical) galaxy scaling relationships better than they do globular cluster ones. But perhaps the most intriguing thing is that at least some UCDs seem to have mass-to-light ratios which suggest lots of dark matter, just like dwarf ellipticals (globular clusters seem to have essentially no dark matter) … so perhaps UCDs are not dwarf ellipticals because they are so close to massive cD (giant elliptical) galaxies?

This very interesting paper by Michael Hilker et al includes some very interesting scenarios as to how the UCDs form, including the UCDs being remnants of the centres of galaxies, or the result of  globular clusters merging or that they are indeed dwarf galaxies! The paper also researches whether UCDs have dark matter haloes or not.

The Distant Globular of Camelopardalis

This week’s OOTW features JeanTate’s OOTD posted on the 22nd of September.

An extragalactic globular cluster

This object has the inventive name of F46 and it lives in the constellation Camelopardalis. It’s not a star, but is in fact a cluster of stars; a globular cluster. It’s not in our galaxy unlike most globular clusters we observe in the night sky, but lies 11 million light years away in the outskirts of NGC 2403; a spiral galaxy that William Herschel discovered in 1788.

NGC 2493

Globular clusters are balls of thousands of old stars gravitationally bound to each other. They orbit their galaxy around the centre, but instead of following the normal path that most stars take – such as in the disks of spiral galaxies – they orbit their galaxy in the galactic halo, which stretches out farther than what is visible in the image above as a sphere, placing the globulars as much as 100,000 light years away from the centre.

F46, being magnitude v 18, is the brightest globular in NGC 2403. But there are plenty more unseen (as far as I can tell) in this SDSS image which you can see in this lovely Hubble image here. Our own galaxy has around 150 or so globular clusters, but many more galaxies have a much higher number; elliptical galaxies for instance have thousands!

You can find many beautiful images and spectra in the MAST database here, and at the Hubble Legacy Archive here!

The Spiral of LL Pegasi

This weeks OOTW features Alice’s OOTD featured on the 16th of September 2010.

LL Pegasi

IRAS 23166+1655

This fuzzy cloud lurking in Pegasus may not look like much on the SDSS, but the Hubble space telescope reveals something very beautiful indeed:

Spi Nebula

Credit: ESA/NASA & R. Sahai

This is IRAS 23166+1655, it may look like a perfect spiral galaxy but it is in fact a pre-planetary nebula, which is a brief burst of nebulosity when a star is in the red giant phase of its life, just before it sheds all of its layers into the interstellar medium as a Planetary Nebula. The cause of the nebula, a carbon star called LL Pegasi, is shrouded by the thick shells of dust and gas that surrounds it and another star, which is gravitationally bound to the other.

As the binaries rotate around each other, making a full turn every 800 years, LL Pegasi throws off beautiful shells of dust and gas on its way round, and because the star throws off the material at different times, it creates the lovely spiral which spreads out for 800 light years! Alice writes in the OOTD that the nebula isn’t lit up by the stars, but is in fact lit up by the galaxy, with one side being brighter than the other because it’s nearer to the plane of the galaxy! 😀

Alice quotes a lovely limerick from Zooite Djj, which I just have to include here:

That nebular spiral is curious
(Though the ‘planetary’ label’s quite spurious)
And I’m losing much gas
Which is part of my mass!
So I tell you, it’s making me furious!

You can read more about the spiral here!

Exploding Eskimos

This weeks OOTW features Rick Nowell’s OOTD posted on the 10th of September 2010.

On January 17th 1787, William Herschel was observing in the constellation Gemini, the result of which led to this beautiful object being discovered:

NGC 2392

NGC 2392; credit: NASA, ESA, Andrew Fruchter (STScI), and the ERO team (STScI + ST-ECF)

This lovely object is NGC 239 and it lies around 2,870 light years away from Earth. It’s a Planetary Nebula, though they don’t actually have anything to do with planets. They come in all different shapes and sizes, from perfect spheres to the intricate one above, which is nicknamed rather appropriately as the Eskimo Nebula 😀

In the middle of all these Planetary Nebulae are the culprits; a core of a low mass star below 9 solar masses. The star at the centre of the Eskimo nebula, at the end of its life when it had became a red giant, threw off all its layers until only its core was left, going from a star much like our sun to a tiny white dwarf. The patterns of ionized gas you can see in the image are around a light year across! This object will remain visible in our skies for a few thousand or so years until it fades away, its gas spreading out into the interstellar medium and contributing to the formation of new stars.

The Stellar Nursery of Perseus

Today’s OOTW features Alice’s OOTD written on the 2nd of September 2010.

SDSS Version of 4191

SDSS Version of 1491

This is NGC 1491, a HII region lurking in the Perseus constellation just above the star Lambda Persei.

HII regions are what they say on the tin; they are made of of hydrogen gas, a considerable amount of which has been ionized by radiation coming from the shorter wavelengths of the electromagnetic spectrum.

HII regions are ionized after a nebula has finished forming a new batch of stars; the regions that shroud the stars get blown away by the winds given off by the stars, creating the bubbles and filaments in the nebula. As these stars emit huge amounts of ultraviolet  light they ionize – meaning the ultraviolet light shoves off the electrons from the atoms that make up the hydrogen gas – everything surrounding them, heating up the clouds of nebulous material and lighting it up, creating great eye candy for us!

Alice gives us different views of NGC 1491 in her OOTD, such as this lovely one:

Credit: Peter Jackson and Rena Smith/Adam Block/NOAO/AURA/NSF

Credit: Peter Jackson and Rena Smith/Adam Block/NOAO/AURA/NSF

I couldn’t resist having a go at playing around with the SDSS fits files on DS9 so I put this image together using I, R and G fits files:

NGC 1491

NGC 1491

Unfortunately I couldn’t find any files that give the full view of the nebula!

And to quote Alice:

Why a nebula? Because nebulae might well soon be on the menu. They’re part of Project IX, whose exact definition is still . . . well . . . nebulous! Well, let’s collapse those clouds and make it less so. Can you suggest a name? They’re looking forward to hearing from you!

Hubble's View of NGC 4911

This week’s OOTW features an OOTD by Alice written on Thursday 12th of August.

NGC 4911

With a redshift of 0.027 this spiral galaxy lies 320 million light years away from us. It’s NGC 4911, a spiral galaxy in the Coma Cluster; a city of galaxies gravitationally bound to each other in the constellation Coma Berenices. LEDA 83751 – the larger elliptical overlapping the galaxy – is actually sat in front of the spiral, which isn’t the best situation for overlap hunters:

Overlapping galaxies are especially useful to Bill and other astronomers interested in dust – the background galaxy acts like a torch, showing what the dust is doing in the former one. The best situation is an elliptical being further away than a spiral, since spirals tend to be dustier and more interesting. Sadly this pair appears to have the bad manners to be the other way round. How rude :D.

– A quote from Alice’s OOTD.

Hubble image of NGC 4911

A new Hubble image of this galaxy has been released showing in more detail the huge amount of star formation going on nearer to the nucleus of the galaxy, the dust lanes streaking their way around the beginning of its spiral arms,  and the wispy spiral structures wrapping their arms around the bustling galactic centre.

Peas Through a Lens

This week’s OOTW features today’s OOTD by Budgieye.

SDSS J001340.21+152312.0

SDSS view of SDSS J001340.21+152312.0

This yellow fuzzy galaxy is a Quasar 1.59 billion light years away from Earth in the constellation Pegasus; it’s just above to the left of the star Gamma Pegasi.

When you zoom in with the Keck observatory you’re treated to this beauty:

SDSS J001340.21+152312.0

Credit: F. Courbin, G. Meylan, S. G. Djorgovski, et al., EPFL/ Caltech/WMKO

Now what the Keck telescope can see and the Sloan telescope can’t are the two red smudges in the blue glow of the Quasar. These smudges are in fact one Pea gravitationally lensed by the QSO sitting in front of it! This is the first ever example of a Quasar strongly lensing an object. This is where a galaxy or a cluster of galaxies are so massive that they bend space-time so much that it visibly bends light around them. So the light emitted by an object sitting behind a cluster of galaxies gets bent around the cluster, creating multiple images of one object.

So how can we tell they are multiple images of the same object?

A quote from Budieye’s OOTD:

To ensure that the two red objects on each side of the quasar is actually the same object, each object must have their spectrum taken separately.
Both blobs of red light had identical spectra, indicating that both blobs are the same object, and that the quasar is bending the light from the distant galaxy into two blobs.