BAN #463: The gorgeous cluster Terzan 4
September 19, 2022 Issue #463
[Hubble image of NGC 3603. Credit: NASA, ESA, R. O'Connell (UVa), F. Paresce (NIA, Bologna, Italy), E. Young (USRA/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]
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Because I am a nerd I follow a ton of astronomy accounts on social media, and also get press releases via email (to the tune of dozens a day; parsing them can be a chore sometimes). Twitter is great for this, because I’ll be doomscrolling through the usual blood-pressure-amplifying manufactroversies and then suddenly some spectacular image of a deep sky object pops up, and my blood pressure drops somewhere close to normal range.
Like, for example, this recent shot of Terzan 4, which made me laugh ruefully at least twice.
[Terzan 4, a staggeringly beautiful star cluster. Credit: ESA/Hubble & NASA, R. Cohen]
Terzan 4 is a star cluster, ostensibly (a word I will get back to, I promise!) a globular cluster. Regular readers will know I love me a globular, especially Hubble images of them like this one (click here to see the full-res glory of it). They’re fun to observe even with small telescopes, looking like slightly fuzzy stellar beehives, and with bigger ‘scopes they look like, well, Terzan 4. Spectacular.
We know that most of them are extremely old, 12 billion years, but some are a little younger. By studying the stars in them we can determine their distance, and combining that with the brightness of the stars we can measure things like how luminous the stars are. One set of Hubble observations were made to do just this.
One paper using Hubble data found the distance to Terzan 4 to be 26,000 light-years. Now, Terzan 4 is almost exactly in the direction of the Milky Way’s center, which is… 26,000 light-years away. Oof. It’s actually a few degrees in the sky away from the galactic center, which translates into it being less than 2,000 light years from the center. Very close.
But there’s more! The colors of the stars can tell us more, too. How? Well, let me go off on a tangent for a few paragraphs.
When you see lots of colors in an image you can bet that the astronomers used two or more filters in their observations. Our eyes have cells called cones that are sensitive to different parts of the visible light spectrum: Some are more sensitive to red, others green, and others yet blue. Every color you can see is due to the way the cones see light; even colors like yellow or orange are detected by your eye due to a combination of the signals sent by the three different kinds of cones.
That’s why we use RGB colors in cameras. Many use detectors that have three pixels bundled closely together, each seeing either red, green, or blue light, and together they make color.
Astronomers can do this after the fact. The detectors themselves are sensitive to a broad range of colors (technically we call these wavelengths, say here from blue to red), and we use filters to select out each color we’re interested in. Some filters mimic our eyes’ cones, while others are centered at different colors. Some filters let through a broad range of wavelengths while others only let through a very narrow slice of them.
OK, so why all this detail on colors? Because stars change colors as they age. A star with twice the mass of the Sun will glow blue for billions of years. When it runs out of fuel ion its core it swells up into a red giant, then blows off its outer layers and become a white dwarf (which glows bluish for along time too). How long this process takes depends on how massive the stars is, with more massive stars dying first, leaving lower mass stars behind.
If you measure the colors of the stars in a globular cluster you can see that stars above a certain mass are all gone. The most massive stars might be ones less massive than the Sun. You can measure that mass from the star colors. We know how long it takes stars to age, so by looking at the most massive stars in the red giant stage, we can get the age of the cluster!
That’s why filters are used; so that the astronomers could select the colors they wanted to see. But there’s more!
Terzan 4 is extremely close to the very center of our galaxy, and the downtown Milky Way is lousy with dust, grains of rocky or sooty material, which are extremely efficient at absorbing visible light. Terzan 4 is actually very faint in visible light because of this, and the Turkish-Armenian astronomer Agop Terzan (what a cool name!) didn’t even discover it until the 1960s.
However, infrared light can get through dust more easily. So the astronomers who observed it with Hubble chose filters mostly in the infrared so they could see it better. Bonus: The red giants put out a lot of infrared, so they’re easier to spot, too.
What you see as blue in that image above is actually light that would appear orange by eye. What’s displayed as green is actually infrared light at a wavelength 1.1 microns, well outside what your eyes can see. Red is actually 1.6 microns. That part confuses people: We can take observations in a lot of different filters, and choose what colors they will be displayed as. That helps us see with our own eyes colors we cannot see. It’s not cheating — I get complaints from people about stuff like this — it’s just a different way of taking data and making it accessible to ourselves.
[Detail near the center of Terzan 4 from the full-resolution image. Credit: ESA/Hubble & NASA, R. Cohen]
So, armed with all this info, look at the image again. You can see lots of blue stars in the center, but those are actually reddish stars. But the ones that look red are super-red: Infrared. You can see more of them off to the right, and that’s likely due to there being thicker dust there blocking the visible light and letting infrared through (another globular, Terzan 9, has the same effect).
You have to account for that dust, correct for it, to get the stars’ true colors. That was done, and here’s where I got chagrined.
I’ve written about this many times, but a lot of globular clusters aren’t. I mean, they’re not actually globular clusters but instead the leftover cores of small galaxies that have been eaten by the Milky Way. If a small galaxy passes through our own, a lot of the stars can be stripped away, leaving behind just the core. I just wrote about this, in fact, in a recent BAN!
Globular clusters tend lose all their gas quickly after they’re born because they lack the gravity to hold on to it. That means that many of them could only make stars for a short time before running out of building material. But small galaxies make stars for a long time before getting eaten, so their star colors are different even though they look very much like globulars.
That’s why I laughed ruefully when I first saw this shot of Terzan 4. The Hubble site says it’s a globular, but I happen to know that, for example, Terzan 5 is a stripped galaxy! So I wondered if the site was wrong and Terzan 4 was like Terzan 5.
Then I saw this paper, and laughed ruefully again. Looking carefully at the star colors, astronomers found that Terzan 4 is very low in heavy elements like iron. That’s a dead giveaway that it’s a very old globular cluster. Massive stars die and explode, creating heavy elements and seeding future generations of stars with them. So small galaxy core stars, which are created over long stretches of time, benefit from this and have these heavy elements — we say they are “metal rich”. Globular clusters, which make one generation of stars (or very few generations) don’t get heavy elements in them. We say they are “metal poor”.
Terzan 4 is metal poor. It really is a globular. So many globulars have been discovered to be partially digested galaxies that I had to laugh. I assumed it was the latter, but it ain’t. It’s a globular.
If you’ve made it this far, well, thanks. You now understand a little better the science behind the images like this, or at least have a better appreciation of how complicated it can be to extract science from images like this. It’s never easy, and you really have to understand what your telescope and filters and cameras are doing before you can have a prayer of understanding what your target object is telling you.
There is considerable overlap of art and science here; displaying an image in such a way that we can understand it better that also produces something of surpassing beauty.
I do so love it when that happens.
What I’ve recently written on the blog, ICYMI
[Saturn’s rings and its tilt may be due to a moon long since destroyed. From Friday’s article. Credit: Photo: NASA / JPL / Space Science Institute / GordanUgarkovic]
Monday 12 September, 2022: A new way to find old impact craters: Look at the burned plants
Tuesday 13 September, 2022: Perseverance rover reveals Martian lakebed is surprisingly volcanic
Wednesday 14 September, 2022: Keck and JWST team up to watch as massive stars blast away gas in the Orion Nebula
Thursday 15 September, 2022: A telescope the size of the Earth sees the wobble of a bizarre planet in a binary star
Friday 16 September, 2022: Could the destruction of a large icy moon explain both Saturn’s tilt and its rings?
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