A galaxy in perspective

One of the things that’s very difficult to appreciate is the relative sizes and distances of celestial objects. After all, their sizes and distances aren’t remotely small enough for us to appreciate. As a result, everything we see looks in some way compressed – one object in an image may just as easily be twice as distant, or ten times as near, as another with little way of knowing the difference.

But if we apply a little knowledge to these images, the relative distances seem to jump out at us, like this:

ESO 318-13 as imaged by the Hubble Space Telescope using its Advanced Camera for Surveys. Image credit: NASA/ESA & Hubble

This is a Hubble Space Telescope image of ESO 318-13 taken with Hubble’s Advanced Camera for Surveys. ESO 318-13 is an irregular-shaped galaxy. That is, a galaxy that has no formal structure such as spiral arms or even a well-defined central nuclear region.

It’s a beautiful image, but what I really love about it is that in one shot we can discern a tremendous amount of perspective and depth. Let’s start with the nearby stuff and work our way outward.

The brightest objects are foreground stars in our own Milky Galaxy. Coincidentally the foreground star in the middle right of the image happens to line up with the middle of ESO 318-13, but keep in mind that the galaxy is itself a collection of stars millions of light years beyond.

If you look at the full-resolution version of the image, you’ll see individual stars within ESO 318-13. And if you look toward the right-edge of the galaxy you’ll spot a beautiful face-on spiral tens of millions of light-years in the distance behind the galaxy. For all of their grandeur, galaxies are mostly empty space, and can be transparent enough to allow background galaxies to easily be seen through them.

Pan around the image and you’ll see a lot more galaxies in the background, including a giant elliptical galaxy in the upper right-hand corner of the image. ESO 318-13 is a dwarf galaxy and may contain a few tens of millions of stars. But the elliptical galaxy in the background is a cosmic titan containing several hundred billion suns!

The distances to objects in this image are all relative – there isn’t anything that tells us exactly how far away they are just by looking at them. To get those measurements we need to use indirect methods such as spectroscopy. But in one image we can begin to get an idea of the sheer scale of the cosmos – keeping in mind that this is still a fairly local region of the universe!

A galaxy in a ring

Now here’s something you don’t see every day:

NGC 660, captured with the Gemini Multi-Object Spectrograph on the Fredrick C. Gillett Gemini North telescope on Mauna Kea in Hawai‘i in August of 2012. The optical image, made using g, r, I, and hydrogen alpha filters, were assigned the colors of blue, green, orange and red respectively. The field of view is 9.3×5.6 arcminutes and is oriented 8 degrees clockwise from north at right and east up. The total exposure (integration) time was 1,620 seconds cumulative for all filters.
Color composite produced by Travis Rector, University of Alaska Anchorage.
Credit: Gemini Observatory/AURA Get the 3844×2444 version!

This is NGC 660 as seen by the Gemini North observatory at Mauna Kea, Hawaii and before we go any further, you definitely want to grab the full-resolution version. NGC 660 is an example of a rare polar-ring galaxy – that is, a galaxy surrounded by a ring of stars that rotates over the poles of the galaxy. First we have the main galaxy itself, seen edge-on to us in the middle. The host galaxy has a very thick central bulge, which classifies it as a lenticular galaxy.

Surrounding the central galaxy is a ring of stars and gas. And the ring itself is bursting with star formation! Take a look in that full-resolution version and you’ll see nebulae illuminated by hot young stars, and bubbles blown out by massive stars that went supernova.

Of course, neither the galaxy nor the ring are solid, and both are warped by their mutual tidal forces on each other. Just wow!

So how did this whole thing come to be? Polar-ring galaxies are thought to form in one of two ways: either in a head-on collision or when one galaxy rips apart a passer by, strewing the former galaxy into a ring.

In the merger scenario, one galaxy pierces the heart of another at a right angle, and the “pierced” galaxy ends up as a ring around the intruder galaxy. In such a scenario, you end up with a collapsed core and a burst of star formation, and NGC 660 certainly has both.

But there are some compelling reasons why the piercing scenario may not be the case here. For one, the galaxy and its ring aren’t at right angles to one another, but rather at roughly 45-degree angles. For comparison, here’s NGC 4650, another polar ring galaxy that was probably formed as a result of such a collision:

NGC 4650 – a polar-ring galaxy most likely formed in a collision scenario. Note that the galaxy and its ring are at ~90-degrees to one another. Image credit: Credit:Hubble Space Telescope/NASA/ESA.

Another problem is that in a piercing scenario, the gas gets concentrated into the host galaxy’s central region, while the ring is left largely stripped of this gas. In NGC 660, not only is there a lot of gas and star formation taking place in the ring, but the host galaxy itself is “thick” with gas, and there’s quite a bit of star formation going on in there as well. In addition, ring itself is tilted about 45-degrees. Simulations of piercing mergers don’t get us gas-rich, 45-degree ring that we see.

That leaves us with with another possibility, called tidal accretion. In other words, the host galaxy “shredded” a lower-mass passer-by galaxy into the ring. There’s some compelling evidence for that here. In tidal accretion, you can have a wider distribution of gas both in the ring and in the host galaxy itself, allowing for the star formation we see in both. It is also possible for the ring to be oriented at any angle with respect to the host, since you don’t have to start out with a right-angle collision. Finally, in a right-angle collision, you’d expect the pierced galaxy’s nucleus to fall inward toward the host, giving you a “double nucleus” at the center. But NGC 660 only has its original nucleus.

That means that we’re likely looking at the aftermath of a lower-mass galaxy that passed close to NGC 660 long ago, and lost a lot of its stars and gas to the larger galaxy as it passed by. NGC pulled this “chunk” of the passer-by into a ring and both the ring and the galaxy tidally distorted themselves into the shapes we see today.

There’s one more very cool thing going on here, hidden from our view but bright at radio wavelengths is a compact , 32 light-year diameter source near the center of NGC 660. This source is most likely a super cluster of stars in a dense cloud of dust and gas, hidden from view but transparent at radio. This source contains perhaps a few thousand hot, blue youthful stars.

Whatever caused the ring, it led to a lot of star formation in NGC 660, making this already rare polar-ring galaxy even rarer – a polar-ring starburst galaxy! Very cool!

It’s truly amazing how much we can figure out about the origins and evolution of galaxies “just” by looking at them and critically examining their features. The answers don’t jump out at us, but we can take what we see and propose models to explain them. Those models that don’t explain the observations are discarded, while those that do are refined to come up with a picture that better matches what we see. This is how science – astronomy in particular – works!

NGC3344 – a mini-me Galaxy in a spin

I can’t say I ever tire of looking at galaxies – the great islands of the universe home to billions of Suns. but the Hubble Space Telescope’s image of NGC 3344 shows a galaxy that in some ways is a lot like our own. Behold:

Nearby spiral galaxy NGC 3344. Tightly wound spiral arms surround a subtle bar of older stars at the center. In this regard, NG 3344 is a smaller analogue to our Milky Way. Image Credit: ESA/Hubble & NASA. Want the giant 3845×3049 version? Git sum!

The bright stars with the diffraction spikes are nearby stars in our own Milky Way, so let’s get them (figuratively) out of the way and admire the beautiful galaxy beyond. NGC 3344 is 25 million light-years away, making it a cosmic neighbor to our own Milky Way, though it contains about half as many stars.

NGC 3344 is a spiral, like our Milky Way. It even has a subtle central bar, oriented vertically in the image. Our own galaxy has a similar bar at the center, though it is believed to be better defined NGC 3344’s. Because it’s relatively nearby, the galaxy covers a wider region of the sky than Hubble’s camera sees, so we only see about 1/3 of it here.

However, a wider field of view shows that the NGC 3344 has an extended, faint ring of stars surrounding it. It turns out this outer ring of stars is orbiting galactic central point in an opposite direction than the inner spiral arms. It’s not clear why this is the case, but it could be due to the cannibalization of a smaller passing galaxy some time ago.

If you haven’t already, check out the giant 3845×3049 version and admire this galaxy in all of Hubble’s glorious detail!

NGC 1672, a galaxy a-bursting with stars

Galaxies are the great islands in the universe where stars live out their lives. But over the course of a galaxy’s evolution, there are times when it is active with more star formation than others, like this gorgeous example of NGC 1672 courtesy of the Hubble Space Telescope:

NGC 1672
NGC 1672 is a barred spiral galaxy with populations of stars forming in the arms and in the nucleus. Click for the 1280×919 version or get the 5302×3805 version

You really want to click that image to see the large version or, if you’re keen to experience the finer details, get the amazing 5302×3805 version.

You’re welcome 🙂

As I was saying, galaxies evolve over time and go through periods where there is a lot more star formation going on than others. NGC 1672 is one of those active galaxies, with star formation taking place not only in the spiral arms, but also in its nucleus as well:

Dude! Star formation in the nucleus of NGC 1672!

NGC 1672 is a type of galaxy known as a Seyfert galaxy. Galaxies typically have quiet nuclear regions – that is, they are dominated by older stars and have very little activity going on in those parts. Seyfert galaxies are quite different from typical spiral galaxies in that their nuclei are are very bright and are typically active with star formation, which you can easily see in the close-up.

So what’s going on here? The answer may lie in the fact that NGC 1672 is also a barred spiral galaxy. The Hubble image shows the central region of the galaxy but this ground-based image shows NGC 1672 in all of its barred-spiral glory:

Portion of a wide field ground-based image of NGC 1672’s taken by the Digitized Sky Survey 2 (DSS2). The image was rotated to match the Hubble Space Telescope image. Note the bar!

As you can see, the bar is chocka-block of stars, gas, and dust that orbit the core in a highly inclined orbit. In other words, the gas is largely “aimed” toward the supermassive black hole at the very center. This in turn creates an accretion disk around the black hole which makes for a very bright nucleus.

But it also means that there is a lot of moving material in the outer region of the nucleus as well, and that means star formation around the nucleus!

There’s still a lot about barred spirals that we don’t yet know. Our own home galaxy contains a bar as well and barred spirals are not uncommon. Astronomers believe that bars are temporary but many questions remain. How do bars form? How long do they last? When do they form – do the form early in the galaxy’s evolution or late? And perhaps most interesting, why do they form in the first place?

There are lots of other little amazing details in the image, and I invite you to grab the 5302×3805 version and start digging around. The brightest stars are foreground stars that live right here in the Milky Way, but there are lots galaxies deep in the background, some of which can be seen through NGC 1672, like this one:

Dude! A galaxy in the background of of NGC 1672!

Notice the color of this galaxy – that’s not because the background galaxy is really that color, but because it’s blue light is scattered by the dust in NGC 1672 itself, letting the yellow, orange, and red light through, giving the background galaxy a caramel color. Pretty cool!

There’s lots of beautiful gems in this image so dig away!

A Sonata of Supernovae – Music of Stellar Explosions

Supernovae are the most powerful explosions in the universe this side of the Big Bang itself. There are a souple of different ways for stars to go supernova, but Type 1a Supernovae shine with a well-known brightness. Thanks to this characteristic, astronomers can use these explosions to measure the distances to their host galaxies, and figure out cool things such as the expansion of the universe.

They can also be used to create music. Feat your eyes and ears (be sure to go full screen to see the fireworks):

This eerie, hypnotic tune was created by Dr. Alex Parker, an astronomer at the Harvard-Smithsonian Center for Astrophysics. Alex used survey data from the Canada-France-Hawaii Telescope (CFHT) over a three-year period from 2003 – 2006. During this time, 241 Type Ia supernovae were detected in the four star fields surveyed.

Alex assigned each supernova a unique note based on the following criteria:

Volume = Distance: The volume of the note is determined by the distance to the supernova, with more distant supernova being quieter and fainter.

Pitch = “Stretch:” The pitch of the note was determined by the supernova’s “stretch,” a property of how the supernova brightens and fades. Higher stretch values played higher notes. The pitches were drawn from a Phrygian dominant scale.

Instrument = Mass of Host Galaxy: The instrument the note was played on was determined by the properties of the galaxy which hosted each supernova. Supernovae hosted by massive galaxies are played with a stand-up bass, while supernovae hosted by less massive galaxies are played with a grand piano.

The result is a mesmerizing sonata without a rhythm, tempo, or measure, played for us by the titanic destruction of stars in the distant past.