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SYFY WIRE Bad Astronomy

Beautifully dying stars and the scale of the Universe

By Phil Plait
Phil Plait Bad Astronomy VLT_NGC474_PN_distance

What can dying stars tell us about the size and age of the Universe?

Possibly a lot, if a new technique developed by astronomers can be used routinely. It looks in other galaxies for stars like the Sun but which are at the ends of their lives, and from that determine how far away they are. This method has been used before, but what’s new here is a refinement that allows it to more accurately measure their brightness, and also go deeper. Much deeper, looking at galaxies that are over 120 million light years away.

Getting accurate distances to nearby galaxies is a big goal of astronomers. It represents a middle rung of the cosmic distance ladder, the steps we take to measure how far away extremely distant objects are. First we find the distances to nearby stars, then bootstrap that to farther ones, then very close galaxies, then moderately close ones… on and on until we’re observing ones billions of light years away at the edge of the observable Universe.

At that distance we rely mostly on redshift, the fact that as the Universe expands distant galaxies are swept away from us, which changes the wavelength of the light they emit in a measurable way. But to use that we need to understand how rapidly the Universe expands, and to get that we need to know how far away closer galaxies are (ones up to a few hundred million light years). Everything depends on everything else, so the more secure each rung is, the more confident we can be.

There are lots of ways to find distances to these galaxies. For example, we can look at surface brightness fluctuations (how smooth the galaxy looks in images), or at how bright red giant stars in the galaxy appear. These methods are getting better and more accurate, but it’s always nice to have another one to compare to.

Enter dying stars. When a star like the Sun dies, it becomes a red giant and blows a dense wind of gas. Eventually the outer layers are completely lost this way, revealing the hot, dense core of the star, called a white dwarf. The ultraviolet light from such an object lights up the gas, creating a gorgeous object called a planetary nebula.

The Ring Nebula, a barrel-shaped shell of gas around a dying star we happen to see end-on. Credit: NASA / ESA / Hubble Legacy Archive / Giuseppe Donatiello

This is the cool part: Different initial masses of the stars means the white dwarfs they make have different brightness, so the planetary nebula lights up by a different amount. However, there is a theoretical upper limit to how bright such a nebula gets (roughly 640 times as luminous as the Sun). If you can observe a bunch of planetary nebulae in a galaxy, you can plot their brightness and determine where that limit is. By comparing that to how bright the nebulae appear through a telescope, the distance to the galaxy can be found (because more distant objects appear fainter in a mathematical way).

The problem is that planetary nebulae in even nearby galaxies are faint. The method starts to have problems around 35 million light years, and has a pretty firm limit at around 65 million light years.

Or “had”, I should say. The new method promises to nearly double that limit.

Astronomers start with spectra taken of the planetary nebula — basically dividing the incoming light into hundreds of individual colors, thin slices of the spectrum. Gas in a nebula, like oxygen, glows at very specific wavelengths, like 500.7 nanometers (in the green part of the spectrum), and is a pretty strong emitter. This helps identify it over, say, a star-forming gas cloud, where hydrogen dominates the glow (at 656.3 nm, in the red).

The idea is to subtract an image of the nebula in a wavelength very close to the oxygen light, where the nebula doesn’t emit much light. This eliminates a lot of stars, background galaxies, and other objects that interfere with the nebula. Once that’s done the nebula appears much more cleanly, and its brightness can be measured. Using the MUSE camera — a very sensitive detector that can get spectra from multiple objects simultaneously — on the immense Very Large Telescope (an 8.2-meter mirror telescope in Chile), they can see dozens of nebulae in nearby galaxies.

Phil Plait Bad Astronomy VLT_NGC474_PN_distance

They used archived observations of galaxies taken by other astronomers to test their technique. For example, some years ago another team of astronomers observed the galaxy NGC 474 with MUSE to look at stars, but as it happens they found 8 planetary nebulae. Reprocessing these data with the new method, the new work found an additional 7 nebulae for a total of 15. They also were able to get better measurements of the nebulae brightness.

Applying the nebula brightness distance scale, they find the galaxy is 121.9 million light years away (with an uncertainty of about 5–12 million light years). This agrees pretty well with other methods, which is really good considering this galaxy is much farther away than any other galaxy whose distance was measured using this method.

Extrapolating from their work, they think that under exceptional observing conditions they can push this out to about 130 million light years and still be pretty accurate.  

Phil Plait Bad Astronomy NGC474_PN_differential_images

This is important work. We use the distances to nearby galaxies to get the distances to more distant ones, and this in turn tells us how fast the Universe is expanding. That also informs us on the age of the Universe, and that’s critical to know to figure out how the Universe behaves overall. Like the distance ladder itself, one thing in cosmology leads to another, and every link in that chain has to be firm or else we can’t rely on it.

There has been a growing issue in cosmology involving the measurement of how fast the Universe is expanding. Observations of relatively nearby objects (out to about a billion light years or two, so a pretty generous definition of nearby) get an expansion rate that doesn’t agree with observations using extremely distant objects (like the cosmic background radiation, which comes from over 13 billion light years away).

There has been some relief in that tension recently using a method that seems to bring these two numbers closer to agreement, but having other methods available will help. This new method gets just far enough that it will be very useful in bridging this gap as well. If we get better distances to these galaxies within 130 million light years, we can extrapolate out to greater distances, and see what happens then.

Hopefully the numbers will still agree. If they don’t, well. Astronomers will have more work to do then, I suppose.

I’ll keep my eyes open to see if and when this group makes more observations of galaxies, and what their results are. The Universe seems to obey a pretty strict set of rules, and our job is to figure those rules out by taking the measure of the Universe. This new method may put us on firmer ground in how we do that.