What happens when Betelgeuse explodes? | Hacker News Reader

What happens when Betelgeuse explodes?

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124 pointsvarmais50 comments

To get a feel for the tremendous size of that star: https://en.wikipedia.org/wiki/File:Star-sizes.jpg

I feel those images are a little misleading. One might erroneously assume there is a dense surface akin to our star at the outer edge of these massive stars but..

"In the outer reaches of the photosphere the density is extremely low, yet the total mass of the star is believed to be no more than 20 M☉. Consequently, the average density is less than twelve parts per billion (1.119 × 10−8) that of the Sun. Such star matter is so tenuous that Betelgeuse has often been called a "red-hot vacuum"."

https://en.wikipedia.org/wiki/Betelgeuse#Density

Also: "estimates of its mass are poorly constrained, but range from 5 to 30 times that of the Sun". Amazing for having on the order of 16 billion times its volume.

How does that happen? Is its fuel more energetic in some sense or does the gravity-vs-pressure equilibrium simply favor lower densities for a total mass of 20M☉?

You're right on both counts! Betelgeuse is burning helium now (post main-sequence). The higher output of helium fusion inflates the outer atmosphere like a giant floppy balloon.

Those images accurately show size, correct? Stars are big.

M☉ = Solar Mass[1]. Huh.

> astronomers have the worst conventions in the world (for largely “historical reasons”)[2]

You don't say.

[1] http://en.wikipedia.org/wiki/Solar_mass

[2] article

Also, for anyone who has an Oculus Rift (or has access to one), Titans of Space does a really good job of depicting things like this.

Holy cow - the star's circumference is about as big as Jupiter's orbit!

https://en.wikipedia.org/wiki/Betelgeuse#Diameter

Still hard to have a carnal comprehension of these. I had a tiny feeling of vertigo though.

My favorite fact about Betelgeuse is that light takes 1.5 hours to travel the distance of its diameter.

"Betelgeuse is already one of the brightest stars in the night sky, sitting somewhere around the 8th or 9th brightest star in the night sky. (These lists don’t include the Sun, which is somewhat obviously always the brightest object in the sky.)"

A good point, although it is fairly rare to see the sun in the night sky these days.

Depending on your definition of "night sky", I would argue it's incredibly easy to see in the night sky!

The remnents of the supernova that occurred in 1054 is known as the "Crab Nebula", AKA M1 on Messier's catalog.

I remember observing this from my back garden some 10 years ago with a small telescope, but that's quite difficult now due to light polution. It's getting hard for people to see stars in many big cities, let alone fainter objects.

There's a guide (in development) at eyesonthesky.com[1] aimed at newcomers, who may be using poor telescopes in poor atmospheric and lighting conditions. I've been following it quite successfully from my back yard in Seattle. It focuses on the brightest galaxies and nebulae, interesting clusters, and binary stars.

[1] http://eyesonthesky.com/StarCharts/FirstLightGuides.aspx

"Nighttime will be a different story. The brightness of Betelgeuse’s supernova is about the same as the quarter moon."

This will last maybe a couple of weeks, but we don't expect it for 100K years.

No, but with billions of stars in our own galaxy, and more beyond, wouldn't (my misunderstanding of) the probability of witnessing a past supernova event during one's lifetime be higher than it feels like it is?

Not really. There are a huge number of stars, but most of them are moving away so fast that the light from their death wouldn't ever reach us, and their light would be shifted in to parts of the spectrum we can't see. Similar to Olbers' Paradox - http://en.wikipedia.org/wiki/Olbers%27_paradox

> most of them are moving away so fast that the light from their death wouldn't ever reach us

Correct me if I'm wrong, but I believe that's mistaken. Given enough time, even things moving away from us at the speed of light will eventually be seen by us. http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope

> ...galaxies that are more than approximately 4.5 gigaparsecs away from us are expanding away from us faster than light. We can still see such objects because the Universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated, there will never come a time that we will see the light from such objects being produced today (on a so-called "space-like slice of spacetime") and vice versa because space itself is expanding between Earth and the source faster than any light can be exchanged.

https://en.wikipedia.org/wiki/Metric_expansion_of_space

The rate of expansion of space-time is currently accelerating so the ant on a rubber rope analogy does not hold.

Edit: from the ant on a rubber rope article you linked

> However, the metric expansion of space is accelerating. An ant on a rubber rope whose expansion increases with time is not guaranteed to reach the endpoint.[3] The light from sufficiently distant galaxies may still therefore never reach Earth.

This is complicated. Given the accelerating expansion of the universe, there's an eventual "horizon" where light beyond that limit will never manage to get here, simply because the empty space between here and there will expand faster than the light can make up the distance. My understanding is that under current cosmological models, a likely scenario is that eventually (on the order of multiple billion years), everything beyond our local gravitationally-bound group of galaxies (us, Andromeda, and a few hangers-on) will be 100% invisible forever. Kinda makes you wonder what distant-future astronomers might think of the old tales of "galaxy clusters" or "cosmic structure formation".

Moreover, A good number are in the Milky way or the Andromeda galaxy, which is bound to collide with ours; plus the respective satellite galaxies of the Milky Way + Andromeda.

Will Univers' expansion make even them "disappear"?

First of all, the "pastness" of looking out into space doesn't really affect probability of witnessing them.

Second, supernova occur quite frequently: http://en.wikipedia.org/wiki/List_of_supernovae That's not even a complete list, just the "important" ones. The odds of one occurring within our lifetime that is naked-eye visible are really quite good. The odds of it being spectacular much less so. It's likely to just be a point in the sky that wasn't there last week and won't be there next week.

It depends in part on what you mean by "witness" a supernova. We discover supernovae all the time (hundreds a year), but pretty much all of those are too faint to be seen with the naked eye. There's a decent chance of seeing a supernova visible with the naked eye in your lifetime, but a pretty small chance of one as bright as betelgeuse.

The transition of the star to nebula will also permanently alter the appearance of the constellation Orion, which is both exciting and sad.

I like the maxim "this too shall pass." It's easy to think of on a generational level - our fathers' generation will disappear, and so will ours. But on the level of a civilization, it becomes both stranger and sadder. Orion is a shape in the sky that humanity has always been able to see (so long as we've lived in the northern hemisphere), and one day, assuming we still grace this planet in 100,000 years, it will no longer resemble the hunter the Greeks imagined. But then, billions of years hence, all the stars will have realigned, the Milky Way will have combined with andromeda. And beyond even that, the energy will so diminish that the bonds of the atoms will release. This too shall pass.

I agree with you.

I have found that the beauty of the temporary nature of everything (especially when considered in enormous time scales) highlights the beauty of the here and now.. and the need to enjoy it. In fact, while some say not to think of the future and stay in the present, sometimes I find that imagining far into the future is precisely what brings me back to the importance of and focus on the present.

Also, there's a feeling of pure awe I get when trying to imagine the unimaginable scale of the future (for us at least). It's a specific sensation.. I am not even sure awe is the right word to describe it.

This article reminded me of something I read a few years ago. What is it like to be really near a supernova? It's a quick read and entertaining too.

Life (Briefly) Near a Supernova:

http://www.nagt.org/files/nagt/jge/abstracts/Dutch_v53n1.pdf

Any risk of a gamma-ray burst affecting Earth?

Every second of every day :)

Not from a supernova. Gamma-ray bursts are a wholly different phenomenon.

Not all supernovae have a GRB and not all GRBs have a supernova, but most long burst GRBs are thought to be from a supernova, no? I've been out of supernova research for a few years now, but that's what I recall.

Based on my degree in astrophysics from the University of Popular Science Blogs IIRC they're from quasars or something.

So, I definitely didn't study GRBs in any depth, but based on my increasingly vague memories from my degree in astrophysics, quasars don't have much to do with it. All of the major proposed mechanisms I recall for GRBs are on the scale of a star or two (supernovae, neutron star mergers, and the like) whereas quasars are a phenomenon involving supermassive black holes in galactic nuclei. Please, definitely correct me if I'm wrong!

You're correct. It's thought that long GRBs originate in certain kinds of supernovae. (What kinds of supernovae specifically isn't really known.) Short GRBs are thought to originate in mergers between neutron stars.

In the long term, this is always a possibility. Which is why for the future survival of our species, we must venture outside our solar system. Likely even outside our stellar neighborhood.

Other than slightly brighter-than-normal nights, will there be any adverse effects on Earth?

The wikipedia article on near-Earth supernovae [1] suggests that Betelgeuse will be pretty safe because of its distance. But exactly how safe? What about all the delicate electronics that we've sent up to space? I suppose anything that can withstand a daily dose of solar radiation would probably be okay, but are there any specific wavelengths that could cause an issue?

[1] https://en.wikipedia.org/wiki/Near-Earth_supernova

Actually, a supernova would look like an impossibly tiny point source, much smaller than even your average laser pointer. I'm pretty sure the consequences of this haven't been thoroughly explored yet.

I'm not sure the Betelgeuse supernova, at a brightness around that of the full moon, would ignite fires, but I certainly wouldn't want to gaze at it with naked eyes, even at that brightness. IMHO such a light, given sufficient strength, could permanently damage the retina in your eyes.

I doubt it. Even without knowing exact surface power density that can cause damage to retina, the magnitudes discussed here sound far from dangerous.

You should know that even an impossibly tiny point source will get projected to a spot of finite size on the retina. The eye is not perfect - there is always some optical aberration. Plus, since we're talking about a star, there's also atmospheric distortion to think about.

The chances of light from Betelgeuse igniting a fire are very much zero. We're talking about light intensity about 10x lower than moonlight.

Your reasoning reminds me of the danger of retinal damage when observing a solar eclipse - a human's natural inclination to not stare at the sun is sharply reduced because the viewable area is so much smaller than usual. Looking at a thin slice of the sun is as dangerous as looking at the unobscured sun, even if it doesn't feel like it. But I don't think Betelgeuse would be dangerous in this way. It would be more like watching a lunar eclipse, which may be done without safety equipment.

Probably not unless you consider a group of self proclaimed holy men trying to follow it :-)

That said if you had some instruments closer to the heliopause you would definitely be able to detect the shock wave at that distance. It would be faint but should be well above the noise level of other particles.

When I read that title, I assumed it had something to do with Douglas Adams.

You are maybe thinking of the Great Collapsing Hrung Disaster that wiped out all the old Praxibetel communities on Betelgeuse Seven.

Starts-With-A-Bang is a must-follow blog collection for anyone mildly interested in Physics. They're all frequent, up to date, and most importantly, accurate.

semi-related: Betty From Betelgeuse is the name of one of the main characters in my book series The Dread Space Pirate Richard.

I chose Betelgeuse partly as a nod to HHGG and partly for the way it read and sounded when pronounced aloud. I think of all the star names it is possibly the most beautiful and memorable.

I'm pretty sure Ford Prefect would be rather disappointed that his home planet got blown up...

Don't worry, he won't know for about 600 years. And they can always construct a new one.

All that said, Betelgeuse isn’t expected to explode for another 100,000 years or so.

Total click bait title.

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