Aldebaran, seen deeply: A glimpse of the Sun’s future

Subscribers grab the cosmic bull by the horns

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A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

If you go outside after sunset over the next few weeks when the sky is fully dark and face east, you’ll see a delightful array of objects in the sky. Ruddy Mars shines brightly, since Earth is lapping Mars around the Sun right now and the two planets are at their closest.

Orion hangs over the horizon, bringing with it the familiar winter constellations. Among them is Taurus, the bull, characterized by a V-shaped collection of stars representing its horns. These stars are mostly part of the Hyades, an actual star cluster — hundreds of stars physically bound by gravity located about 150 light-years from Earth. On a galactic scale that’s practically on our doorstep.

The brightest star in Taurus is Aldebaran, located at the tip of one of the horns. Weirdly, it’s not part of the Hyades! It’s about 65 light-years away, and only coincidentally aligned with the more distant cluster.

It’s a luminous star, shining 400 times more energetically than the Sun, so even at that distance it’s one of the brightest stars in the sky; for comparison if the Sun were that far away it would barely be visible to the eye.

My friend and amazing astronomer/astrophotographer Adam Block thought it might be interesting to point a telescope at Aldebaran and take a very deep image. And so he did: He took a total exposure of 15 hours, 5 hours each in a red, green, and blue filter to create a staggering image of this brilliant star and the area around it:

Wow. He has a 3,000 x 3,000 pixel version also that you would do well to look at!

There’s a lot to see here. First, let’s focus on the star itself.

Aldebaran is a red giant, a star that was once much like the Sun, similar in mass and size, but long ago ran out of hydrogen fuel in its core to fuse into helium. The helium has built up like ash in a fireplace, compressed into a tiny ball roughly the size of Earth, and fiercely hot. So hot, in fact, that hydrogen is fusing into helium in a thin shell around the helium core. This generates a lot of energy, far more than the star made earlier, which is blasted outwards into the layers of gas surrounding the core. All that energy makes the outer layers swell up and cool, turning the star into a red giant. Aldebaran is currently about 50 times the diameter of the Sun!

I go into this more in my episode of Crash Course Astronomy on low-mass stars:

So, in other words, Aldebaran is in the process of dying. The shell hydrogen fusion is adding helium to the core, which will eventually get so hot and dense that helium fusion will ignite, creating oxygen and carbon. This will cause the star to contract and heat up, but as the carbon and oxygen build up in the core the star will produce more energy again, swelling back into a red giant. It doesn’t have enough mass to squeeze the carbon and oxygen hard enough to make them fuse, so this is the real beginning of the end. It will blow off its outer layers, exposing the core to space, hen becoming a brief planetary nebula before fading away entirely.

But all that’s still in the future. Interestingly — and I wasn’t aware of this until reading up for this article — a planet has been found orbiting Aldebaran! Called Aldebaran b (or more formally Alpha Tauri b, since Aldebaran is the brightest star in the constellation and called Alpha Tauri), it’s a super-Jupiter with more than 6 times the giant planet’s mass, and orbits the star every 623 days. It’s roughly as far from Aldebaran as Mars is from the Sun, so it must be getting cooked by the red giant.

Another thing I didn’t know before researching the star for this article is that it may be a binary, with a second star orbiting it. Several faint stars are located near it in the sky, and one appears to be at the same distance and moving though space along with it, implying they’re together. That’s hard to prove though, since the star is a red dwarf and thousands of times fainter than the red giant, making observations difficult.

You can see a lot of what looks like wispy patches of dust floating around in the image, too, and those are, well, wispy patches of dust. Cosmic dust: Grains of rocky and sooty material expelled from previously existing stars as they died. This area of the sky is home to the Taurus Molecular Cloud, a cold, dense cloud of hydrogen and other gases about 430 light-years from Earth (so, well on the other side of the Hyades). It’s the nearest active star-forming cloud to Earth, and is heavily studied for that reason. But it also means that part of the sky is lousy with dust. That dense clump to the upper right of Aldebaran looks to me to be forming stars in it now; the red fuzz is possibly hydrogen gas glowing due to newborn stars zapping it with energy.

And finally, the lovely blue stars you see in Adam’s image are likely members of the Hyades itself. Most are more massive and hotter than the Sun (which is why they’re bluer), but fainter than Aldebaran because they may be intrinsically less luminous and are also farther away, dimming their brilliance.

But clearly, clearly, Aldebaran is the star of the show in Adam’s amazing shot.

Look closely again at Aldebaran in the image, and if you can, with your own eyes outside. When you gaze upon it, you are seeing a later chapter in the Sun’s story, some six or more billion years from now.

Astronomy is like time travel. Usually we see the past, since it takes years, centuries, even millions or billions of years for a distant object’s light to reach us. But sometimes it allows us to see our own future, too.

Et alia

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