Hubble Portrait of a Beautifully Odd Spiral Galaxy

The entire universe in blog form
Nov. 26 2013 8:00 AM

Beauty Is Only Universe Deep

Galaxies come in many shapes and sizes. You can make broad categories for them — spiral, elliptical, irregular, peculiar — but even within those bins there’s a lot of variation. Even more confusingly, they look very different depending on how you look at them: what telescope you use, what colors of light you use, and how you assemble the various observations into a single image.

NGC 4921 is a good, if spectacular, example of this. It’s a face-on spiral located about 300 million light years away, deep in the huge Coma Cluster, a vast collection of galaxies with over a thousand citizens.

NGC 4921
Hubble's grand view of the magnificent face-on spiral NGC 4921. Click to galactinate, and you want to.

Photo by NASA / ESA / Roberto Colombari / Hubble Legacy Archive


That Hubble image is breathtaking! It was put together by Roberto Colombari using observations from 2006 and 2007. It’s not really true color; it’s a combination of observations using two filters; one that lets through light in the red to the near-infrared (shown as red in the image), and the other that lets through light from the blue-green to red (shown as blue in the image). He also created a “pseudo-green” color by combining the blue and red images. This doesn’t add any real information, but helps produce a balanced three-color image that’s prettier.

Still, there’s a lot to see here. The most obvious feature is the spiral; the arms are clear though weaker than in most such galaxies. This is common with spiral galaxies in dense clusters; as they move through the cluster environment, gas between the galaxies can strip a galaxy of its own internal gas, much like you can roll down a car window to flush out the inside air as you drive down a highway. This ram stripping, as it’s called, can suppress the formation of stars, which in turn dampen the strength of the spiral arms themselves. Observations using different telescopes confirms that NGC 4921 is undergoing this sort of process. There is some ongoing star formation — you can see that as clumps of blue, where hot, massive stars are being born — but most spiral galaxies have a much stronger signal of this.

The next obvious bit is the complex structure of dust within the galaxy. This material blocks the light from stars behind it, so we see it as dark filaments twisted around the galaxy. I was surprised to see it popping out in this image, and all at roughly the same distance from the center. Rings of material like this sometimes happen after a galactic collision, or a near pass from another decent-sized galaxy. Again, that’s common in clusters.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Also, you can see a lot of distance background galaxies as well. Most of these are much farther away, though some are also Coma Cluster members. NGC 4921 looks translucent in this image, actually, but again that’s because it’s so low in dust and gas. You really can see right through it!

Hubble has observed this galaxy many times, because it has a special kind of star in it called a Cepheid variable. These stars pulse at a fairly predictable rate, and that rate is related to their actual, physical luminosity: how much energy they emit. That means that we can measure how bright they appear to be, and use that to determine their distance! This makes them benchmarks in space, allowing us to figure out how far away galaxies are.

NGC 4921
A slightly different view of NGC 4921. Click to embiggen.

Photo by NASA, ESA and K. Cook (Lawrence Livermore National Laboratory)

This image is a combination of red/infrared observations and some taken using a blue filter (and again, a pseudo-green channel made by combining the two). It looks very similar to the new image, but as you can see the colors are a bit more washed out. The dust isn’t as obvious, nor are the star-formation regions. That’s all more an artifact of the filters used, but still, it shows that a galaxy can appear quite different just due to how you look at it.

These images of NGC 4921 are a good — and very beautiful — reminder that what you see is not what you get. There are many ways to see things, and they can all tell you something different and important. You have to be aware of that, and to know how to interpret it. Otherwise, how will you ever hope to understand the magnificent Universe around you?


Medical Examiner

Here’s Where We Stand With Ebola

Even experienced international disaster responders are shocked at how bad it’s gotten.

It’s Legal for Obama to Bomb Syria Because He Says It Is

Divestment Is Fine but Mostly Symbolic. There’s a Better Way for Universities to Fight Climate Change.

I Stand With Emma Watson on Women’s Rights

Even though I know I’m going to get flak for it.

It Is Very Stupid to Compare Hope Solo to Ray Rice

Building a Better Workplace

In Defense of HR

Startups and small businesses shouldn’t skip over a human resources department.

Why Is This Mother in Prison for Helping Her Daughter Get an Abortion?

How Ted Cruz and Scott Brown Misunderstand What It Means to Be an American Citizen

  News & Politics
Sept. 23 2014 12:43 PM Occupy Wall Street How can Hillary Clinton be both a limousine liberal and a Saul Alinsky radical?
Sept. 23 2014 2:08 PM Home Depot’s Former Head of Security Had a Legacy of Sabotage
Sept. 23 2014 1:57 PM Would a Second Sarkozy Presidency End Marriage Equality in France?
  Double X
The XX Factor
Sept. 23 2014 2:32 PM Politico Asks: Why Is Gabby Giffords So “Ruthless” on Gun Control?
  Slate Plus
Political Gabfest
Sept. 23 2014 3:04 PM Chicago Gabfest How to get your tickets before anyone else.
Brow Beat
Sept. 23 2014 2:31 PM 3 Simpsons Showrunners Reflect on New Fans and the “Classic Era” Myth
Future Tense
Sept. 23 2014 1:50 PM Oh, the Futility! Frogs Try to Catch Worms off of an iPhone Video.
  Health & Science
Sept. 23 2014 1:38 PM Why Is Fall Red in America but Yellow in Europe? A possible explanation, 35 million years in the making.
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.