Stars in the Andromeda Galaxy find the fountain of youth


This science+comics post takes us where this blog has never gone before:  to outer space.

Phil Rosenfield is a PhD candidate at UW in the department of Astronomy, and studies stellar and extra-galactic astrophysics (woah).  He uses data collected on the Hubble Space Telescope to look at stars in distant galaxies and is particularly interested in how stars change over time.

One of Phil’s interests is in the Andromeda Galaxy.  Here’s a photo – breathtaking, isn’t it?

image from, shared under Creative Commons license by Adam Evans

The Andromeda Galaxy, like other galaxies, is a cluster of stars and dust and gas and dark matter.  And get this – it’s the farthest thing you can see with just your eyes from Earth.  Wow, now that’s crazy!  If you go out on a clear night in late summer or early fall, you can find it for yourself, using these steps from Phil’s blog.  And while you’re gazing at that tiny speck in the night sky, know that the light hitting your eyeballs from this particular galaxy actually left it 2.5 million years ago – back when our earliest ancestors were just figuring out how to use primitive tools to help them scavenge for food.

Phil and his lab group are working with data collected on the Hubble Space Telescope (a.k.a the HST, if you’re hip with the Hubble lingo).  The HST orbits around the earth about every 97 minutes.  As a scientist, if you want to get the HST folks to take a particular set of images, you need to put together a proposal – usually with a large group of scientists who are interested in the same thing.  Julianne Dalcanton, the head of Phil’s lab group, and several other scientists, successfully convinced the folks at Hubble to capture super-detailed images of the Andromeda Galaxy.  These images are being collected over a total of 828 HST orbits over the course of four years.

The Andromeda Galaxy is shaped sort of like a spiral-y pancake, a flattened disk with a bright bulge in the middle.  New stars tend to form in the disk part of a galaxy, and closer to the bulge you’d normally expect to see older stars. So how do you know whether a star is young or old, just by looking at it?  The main way to tell the difference is by looking at the frequency of light that is emitted.  Young, massive stars are mostly emitting light in the ultra-violet (UV) part of the light spectrum – if you picture the violet part of a rainbow, ultra-violet would be the next color past violet, which our eyes can’t even pick up. Older, not as massive stars, on the other hand, emit most of their light in redder parts of the spectrum, like our sun, which emits most of its like in optical frequencies.

The first zoomed-in image that the HST collected of the Andromeda Galaxy was near the central bulge.  As an astronomer, you would not expect to find stars emitting UV light from so near the bulge.  But when Phil’s group started doing analysis on the image, they were surprised to find that there were LOTS of stars near the bulge with the characteristics of young stars.  But they couldn’t be so young so near the bulge – star formation doesn’t happen there.

To explain this discrepancy, astronomers have figured out that there must be a completely different type of star.   Here’s a figure describing the “normal” life history of a star, and then the life history for the “new” type of star.  Here’s a figure comparing the two paths that a star might take, if it started out about the same mass as our sun.


Learning about what is happening in a nearby galaxy like Andromeda can help astronomers understand where the light is coming from in more distant galaxies where even the HST can’t make out individual stars. If Phil’s group can confidently say how old stars like these are, then they can use that knowledge to help them interpret light from other galaxies that have this strange amount of UV light in their centers. They can then begin to address some big questions, like: how and when did galaxies form? And how old is our universe? The results of this analysis give astronomers a way to independently test other methods that are used to answer these kinds of questions.

If you want to know more about Phil’s work, check out his blog:, especially his post on Bright UV stars in the bulge of M31 (Andromeda Galaxy).

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