A bursting star in a starburst galaxy

I am endlessly amused by image and/or press releases. I get a lot of them (as a professional science communicator I subscribe to quite a few mailing lists), and it’s interesting to see how they frame the news. Sometimes, it’s a big discovery. Sometimes, it’s a bit of news about progress on a piece of equipment or mission. And, sometimes, it’s just a tidbit about a pretty picture.

I’m all for that last; a pretty picture is, well, pretty, but it can also be used to talk about some fun science in addition to serving as a bit of eye candy. Beauty and science often intertwine, so using a cool photo as an attention grabber to open a pathway to even a somewhat deeper understanding is fine by me. But I’m sometimes tickled by the choices made in the releases.

For example: The good folks at the European Space Agency just released a gorgeous photo of a nearby spiral galaxy taken by Hubble. Feast your eyes on this:

Wow! That’s NGC 4536, a galaxy 48 million light years away and roughly the same size as our Milky Way. It’s near the huge Virgo Cluster of galaxies, a collection of well over a thousand galaxies about 50 million light years away. NGC 4536 may not be a part of the main cluster, but instead dwells in a subgroup of it called the M61 group a bit to the south.

The image isn’t “true color” as such; it’s a combination of light seen through a green filter (displayed as blue), and a near infrared filter (displayed as red). That skews the colors a bit. For example, big clouds of gas tend to emit strongly in red light, and that’s difficult to see here, since the filters used select for colors other than that one. Still, it’s a lovely shot.

Anyway, the release talks about how NGC 4536 is what we call a “starburst” galaxy: one that is undergoing an intense episode of star formation. We see quite a few galaxies like this; sometimes that’s due to a collision with another galaxy. When that happens the gas clouds slam into each other, collapse, and form stars rapidly (and by that, I mean over some millions of years). A galaxy like that can produce stars at many times the rate of a normal galaxy.

That’s very cool, and I was happy to do a little digging to find out more about it, but during my reading, I had to chuckle. One of the first things I did was to look at the listing for the Hubble observations, themselves. NGC 4536 has been a target for Hubble many times, but in this particular case, it was observed using the Wide Field Camera 3 in 2010. Why? Well, that’s the funny thing. It wasn’t observed because it’s a starburst galaxy; it was observed because it was the host to a star that exploded!

In 1981, the light from that supernova reached us. It was a Type Ia, a very special kind of explosion. For details, watch my Crash Course Astronomy episode about them, but in a nutshell we think these explosions all give off the same amount of energy (or can be calibrated in various ways to put them all on equal footing). Because of that, they can be used as distance markers! If we see one that looks fainter, it must be farther away than a brighter Type Ia explosion.

A very important piece of this is to find nearby Type Ia supernovae, so we can use other methods to find their distance as accurately as possible. If one occurs in a nearby galaxy, there are several processes we can employ to get their distance. These don’t work for galaxies farther away (they’re too faint), but if a distant galaxy hosts a Type Ia supernova we can use what we’ve learned from ones in nearby galaxies to extrapolate the distance to that farther one.

Supernovae can be seen from billions of light years, and so if we can measure their distance, we can measure how the Universe itself behaves on vast scales. For example, we know the Universe is expanding. But by measuring the light from very distant Type Ia supernovae, two different teams of astronomers discovered that the expansion rate of the Universe, itself, is increasing. The cosmos is accelerating!

And that’s why NGC 4536 was observed. Hubble can see fainter stars in nearby galaxies than other telescopes, so it was used to look for others kinds of stars that can serve to get the galaxy’s distance (the kind in this case are called Cepheid variables, which are a tried-and-true distance marker). Once the distance to NGC 4536 is found using those, then the Type Ia supernova we saw in that galaxy in 1981 can be used to measure even farther distances. NGC 4536 is one rung on the distance ladder, a way we can go from getting distances of nearby stars out to the vast depths of the entire Universe.

Mind you, I am not in any way trying to put down the release for this image! It’s quite good, and fun to read. The fact that NGC 4536 is a starburst is interesting, and a fine bit of info to discuss when showing the Hubble image of the galaxy. But I found it mildly amusing that the reason the observations were made in the first place is also interesting, and worth explaining.

I guess that’s a problem when you’re trying to communicate science. Which avenue do you explore? A galaxy is hardly ever just a galaxy. It’s a huge community of stars, gas, dust, dark matter, and all sorts of science that makes it wonderfully rich and fascinating. Sometimes you just have to pick one thing and go with it.

Hmmmm, maybe I was wrong. That’s not a problem at all! Or, if it is, then it’s a good one to have.

Correction, April 24, 2017: I originally attributed Jennie Hottle in creating this picture from Hubble images, but that was an error on my part.