Meet Rigel in Orion, a star with supernova potential

Both Betelgeuse and Rigel are potential supernova candidates. The view shows the sky facing southeast around 8 p.m. local time in early January.  Maps created with Stellarium

Everybody’s always worried about Betelgeuse in Orion blowing up as a supernova. There’s a good chance that may happen one day, but no need to panic. The star’s too far away to trouble earthlings with its future fireworks. Opposite Betelgeuse and below the winsome triad of stars that form Orion’s Belt, another potential supernova star sparks and sputters on winter nights – Rigel. The name comes from ancient Arabic and refers to the foot or leg of Orion.

Like Betelgeuse, Rigel (RYE-jel) is also a supergiant star but one of a different color and temperature. Astronomers classify it as a blue supergiant with a surface temperature of over 20,000 degrees, twice that of the sun and 3 1/2 times hotter than Betelgeuse.

Rigel, a blue supergiant star, is 18 times more massive than the sun and 74 times its size. Credit: CWitte with minor alterations by Bob King

At a distance of 860 light years, Rigel is big enough and close enough to have its diameter measured directly. As you might guess, it’s huge – 74 times the size of our sun.

Placed where the sun is now, this stellar beast would extend nearly to the orbit of Mercury. From Earth, Rigel would span 35 degrees of sky and shine at a blinding -38 magnitude. We’re talking a powerful sunburn in a minute or two.

Great distance tames Rigel’s true ferocity as a young, energy gobbling star into a pretty blue-white twinkle reminiscent of sunlight on snowflakes. Rigel shines at 0.1 magnitude or about as bright as Capella in Auriga and Vega in Lyra.

Being extremely hot, blue supergiants burn up their energy stores quickly. At the tender age of 10 million years (young for a star), Rigel has already depleted its core of hydrogen fuel and has moved on to burning hydrogen in a surrounding shell.

If put in place of the sun 93 million miles from Earth, Rigel would cover 35 degrees or sky or about twice the area of the constellation Orion.

Helium “ash” created from hydrogen burning will one day ignite and serve as fuel as will progressively heavier elements like oxygen, neon and silicon over time. Rigel will puff up and redden just like Betelgeuse in those far-off days.

Just before a supergiant star blows it has a core made of iron that cannot “burn” to create energy to push back the force of gravity. Gravity takes hold and the star collapses.

Unfortunately, supergiant stars reach the end of the line once all the remaining silicon fuel has undergone nuclear fusion to create a core of iron. Iron requires more energy to fuse than the energy it releases, so it won’t burn like the other elemental fuels. With no burning to push back against the crushing force of gravity in so large a star, the core collapses and sends out shock waves that rip it apart in a supernova explosion.

Rigel is a close double star in a small telescope. Use 100x and up to split it cleanly. Credit: Fresno State University Observatory

When will this happen? Probably millions of years down the road. Since Rigel’s 300 light years farther from Earth than Betelgeuse, we needn’t worry about it either. Instead, our future descendants should prepare for a wondrous light show. Jim Kaler, professor emeritus at the University of Illinois, estimates that Rigel will become as bright as the half-moon when it finally blows up. Picture all that light concentrated in a tiny point of light. We’d easily see our shadows at night by supernova light!

If you have a small telescope 4.5 inches or larger, point it at Rigel some night. It’s one of the finest, if challenging, double stars in the sky. The 7th magnitude companion peaks out from under the glare of the main star a very short distance (9 arc seconds) to its south. On a night with steady air and good seeing, this pair is a beautiful sight.

29 thoughts on “Meet Rigel in Orion, a star with supernova potential

  1. On your picture of Rigel with the Sun there for perspective, are those the equivalent of Sun spots (Rigel spots?) on Rigel? If so, and assuming the perspective to close scale, then those Rigel spots are considerably bigger than the entire Sun! Man! Can you imagine what a CME would look like from a spot that size? Have we ever been able to ‘see’ (by one means or another) things like CME’s on another star? Is that what makes a variable star vary?

    Is it even possible to have a Goldilocks Zone (I think that’s what it’s called) around a star like Rigel? Even if a planet were at the right distance away from the star for sort of the average right temperature, one CME that size would cook the planet, wouldn’t it?

    Bob: you’re teaching me stuff *everyday* man. Thanks!

    • Bob,
      Yes, those are starspots but don’t take their sizes too literally. Rigel has them but how big, we don’t know. Astronomers haven’t recorded CMEs from other stars but can monitor their starspot cycles by measuring the intensity of calcium emissions in the stars’ chromospheres (layer above the visible surface called the photosphere). Increased emission points to more activity including starspots. Starspots are also detected indirectly by how much they darken the star as they rotate in and out of view. The biggest spots can cover up to 30% of a large star!

    • With Bob’s permission I’ll reply to the second question, as I have as special liking for exoplanets.

      You’re right Bob C that every star has around it an “habitable” (or “Goldilocks”) zone, where temperature is Earth-like.

      And you’re right that few planets were found around big hot stars like Rigel. However one of the reasons is not much the CMEs, that is peak events of solar wind: already the average solar wind (i.e. charged particles), and overall the average electromagnetic radiation (mostly UV and X) from such hot big stars, are intense and “boil up”, precisely scatter away, the protoplanetary disks alfo for nearby stars, inhibiting planetary formation. This process is called “photoevaporation”, and was observed directly in amazing photos like this http://en.wikipedia.org/wiki/Photoevaporation.

      The hot stars in question are those of hot color temperature (which unlike everyday language are toward violet, not red): O (violet/ultraviolet) stars and B (blue) stars – meaning both “main sequence” (“dwarf”) stars (i.e. still young in their life) O&B, and their aged version: the B “supergiants” like Rigel.

      • Thanks Giorgio. The math in that Wikipedia article is, of course, *way* beyond me, but I think I got the jist of it. But then that prompted another question (imagine that!). If those super hot and super big stars are SO hot that they are literally blowing away the remains the disks around them and even the disks around other stars relatively close to them (0.5 light years way – that’s ‘close’ right?), then how did they get *that* big and *that* hot in the first place? The star would have started ‘burning’ (nuclear fusion) long, long before it got that big. And, if I understand that article right, the main fuels (hydrogen and helium) are what get blown out away from the star first. I guess it makes sense that they should get blown back first, as they are lightest elements. But that is also the ‘stuff’ that would make the star grow in size and in heat, isn’t it? How does that work?

        Also, if I may, a question about the Spitzer picture in the article you linked: it provides a legend there showing the distance of 0.5 light years. Is the big star in that picture (it also looks like it might be a very close double star?) really something close to 0.25 light years in diameter? That would be much bigger than our entire solar system, wouldn’t it? And one final question (for now… there will undoubtedly be more!): the smaller star in that picture that is reported as having its disk being blown away, is it possible that it is actually acting much like a comet – being drawn rapidly in toward the larger star and leaving a comet-like tail behind it which would undoubtedly still be largely material from a disk, but now the reasoning for the tail changes significantly (not just outward pressure from the big star, but also significant inward motion from the smaller star)?

        Thanks again to both you and Bob for all your time and effort answering my many questions!

        • I’ll gladly reply your inspiring questions, if done one at a time (and when I know the answer): even a single such question requires space and time for a proper reply in non-specialized language because this is non-trivial physics and there are always precisations to be done. I also still have on hold your questions about my home experiment about perihelion. For the rest I wouldn’t like to interfere too much with Bob K, which administers here and is expert. By the way as always Bob K please correct me if you spot some error in my posts.

          A dwarf star is stable because there is a balance between gravity (which tends to contract the star) and pressure (toward outside) due to nuclear fusion. In an aged star, when core consumed all fuel, the fusion happens in outer shells, where gravity is less intense, or at higher temperature because burning heavier elements, so the star has become giant.

          Two cases must be distinghuished:
          - If the star has moderate mass (for example Sun), by becoming giant it becomes *cooler* (reg giant) because the energy is distributed in a wider region.
          - For O&B (massive) stars the intense radiation indeed pushes matter toward outside, giving a secondary contribution to the star growing in size when it ages. These stars, when aging and growing in size, can cool down to a red supergiant or remain blue (blue supergiant). For these massive stars, both in dwarf and supergiant phases, the radiation pushing matter outside causes also a mass loss (by stellar wind) (with higher rate in supergiant phase), but the rate is not enough to make the star “evaporate”, also due to the high star mass.

          Postscript: In the Spitzer photo, I’m not sure the star “bubble” can be taken as star size – it may be big because of photo over-exposition.

          • But how do these O & B type stars – that’s the very big, super heavy and super hot stars, right? – how do they get to be that big and that hot in the first place if they are ‘blowing’ away the very fuel that causes them to grow? If this star is ‘blowing’ away a disk around another star 0.5 light years away, then it must have been blowing away its *own* fuel (disk of material) long before it got *that* big and hot. Once it had grown just big enough to blow away its own source of fuel, it should not have grown any bigger (more massive) or any hotter. I hope I am making sense here.

          • Bob,
            Mass tells almost everything you need to know about a star. Big stars form from large clouds of pre-stellar gas and dust. The more massive the star, the faster it eats up its fuel supply and the shorter its life. Once a large star forms, it looses material just like the sun through stellar winds. Yes, it looses more but it’s also so big and massive that is has MUCH MORE material to spare. Supergiant stars also pour out a flood of ultraviolet light that boils away material around newly-forming stars, which is then carried off by its powerful stellar winds. Although supergiants can be bullies in their neighborhood, they deplete their fuel in a matter of millions of years (instead of billions like the sun) and leave the scene often as supernovas.

          • Just a question about the burning of the various shells (layers) of a star: If I understand this correctly, when the core has created the heaviest elements that the star can make, then the fusion reactions stop in the core. I have assumed that the heavier elements are constantly ‘sinking’ into towards the center of the star and the lighter elements ‘floating’ nearer to the surface. Here’s my question: is it true that fusion is constantly going on at all layers of the star all the time making heavier and heavier atoms until that star it reaches the limit of what that star can make (ie the iron at the center of the diagram above)?

            Also, why is the hydrogen floating on top of the helium in that diagram (the heavier element on top of the lighter one)?

          • Bob,
            Yes, the star is fusing in multiple layers and the heavier elements – the “ash” as it were – sinks to the core. Hydrogen is the lightest element in the periodic table so it lies above helium.

          • Bob, thanks for your replies below. I don’t have a link to be able to reply to them directly, so I hope this will do.

            First I expect I caused you to smile with my ‘oops’ regarding hydrogen and helium. I guess my grade 8 chemistry from 35 (or so) years ago is a little foggy. I should have known that, but had them the other way around in my mind.

            But to my question, either I must not be asking my question clearly or I am not understanding the answers that you and Giorgio have provided. I understand that stars give off material through their solar wind. But that’s not the point of what I’m asking about. We understand that a star forms from pooling gas and dust drawn together by the gravity of the forming star. At some point there is enough pressure and heat inside that star that nuclear fusion starts. But the gravity of the star is still pulling in more material from the cloud or ring or disk of dust and gas around the star, and thus the star is ‘growing’. At some point, though, if I have understood this right, the star grows sufficiently large enough and hot enough that the solar wind coming off of it actually overcomes the gravity of the star and pushes back (blows away) the disk or cloud of material around the star that has been causing it to grow. At this point, the star should stop growing, shouldn’t it? But the point of the article Giorgio linked was to suggest that there are some stars that are *so* big and *so* hot that they will blow away the cloud or disk of material around *another* star up to 0.5 light years away. But the star should have stopped growing when its solar winds got strong enough that it began to blow away its own disk or cloud of material that had been falling into the star. Surely that should have happened long before it got to the point of being able to blow away the material around *another* star, albeit a very ‘close’ one. And that’s why I began to wonder if there wasn’t something else (or additionally) going on in that picture. My question is “how does a star get to be that big? How can a star continue to grow even after it has blown away its own cloud or disk of material around it?

            Again, both of you, thank you for your patience with me.

          • Bob,
            Even the sun blew the remaining gases from its disk in its youth which continues today as the solar wind. Supergiant stars keep blowing and blowing and blowing with much stronger winds coupled with an enormous output of UV light. Gas clouds come in all different sizes. Giant stars form from giant clouds of gas and dust. Once they’ve formed and nuclear fusion is underway, they stop growing. Tiny dwarfs form from much smaller clouds and also cease to “grow”. Giant stars develop very strong winds and lose significant amounts of mass, but as I wrote earlier, they have lots more material to spare.

          • BobC, to reply your question below, in addition to quoting AstroBob replies, I think the key to clear up your doubt is probably that you should not confuse the grow in *mass* which happens only in protostar (so early) stage, and the grow in *radius* which happens later, mostly in giant/supergiant phase which means near end of star’s life.

          • BobC, of course what I wrote in my latest post just shifts the problem to the protostar stage, when indeed at some point the intense radiation could prevent mass growth. So I now really got your question. I just discovered this Wikipedia paragraph under the voice “Star formation” which answer exactly your question:
            “Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses. Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar [...] Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass”.
            Source: http://en.wikipedia.org/wiki/Star_formation#Low_mass_and_high_mass_star_formation
            Hope this helps and closes topic.

          • Bob and Giorgio,

            Again, thank you both for your time in trying to help me understand this. It is much appreciated! Sorry for the delayed reply as I am back to work now following a nice Christmas break.

            I hope I wasn’t coming across as argumentative. If I was, please forgive. That certainly wasn’t my intention. I wasn’t so much trying to express “doubt” as I was just trying to figure out how that could be so.

            Giorgio, that Wikipedia link was helpful. There is some stuff in there that I don’t understand too (which should come as no surprise!). And in many of the links from that page, the math was way over my head. But perhaps as time goes on and I learn more, then more of this will make sense to me too.

            As you said, Giorgio, the last paragraph did deal with my question. The idea of radiation jets (I assume that means jets from the north and south poles of the star or protostar) is interesting. But if that is what happens, then I wonder why those jets wouldn’t persist. To my simple and rather uneducated mind, the other idea given in that paragraph for how such large stars could form in the first place makes some sense to me and maybe even fits with what we are seeing that Spitzer photo. That is, of course, the idea of the merging of stars and/or protostars. That seems like it could explain how all of the additional mass gets added to these huge stars. In my mind that means that, while the stellar winds of these giant stars are strong enough to blow away clouds or disks of material around themselves and even around incoming large bodies, they are not strong enough to blow away the actual large bodies of dense mass such as whole stars or protostars as they are drawn into the giant star.

            Again, thank you Bob and Giorgio for your time and patience and for your willingness to share your knowledge.

          • No problem BobC, you’re welcome.
            Yes, among the two theories about O&B star formations, the one that they can only form as merging of lighter stars is also my favorite. A very attractive idea, thank you for letting me discover about.

  2. “Everybody’s always”…what a great way to start a story, a blatant and hilarious non-sequitor. BTW, I say betel-GOYZ, and not beetle-juice.

    • Andrew,
      Hah! Yes, must have a bit of fun, you know. I’ve heard that alternative pronunciation for Betelgeuse. I say it as beetle-jooz, so there must be at least 3 ways to say it.

  3. Bob, last week I imaged the star Rigel, hoping to see its companion star. I used .004 seconds of exposure time. After processing the ten files, there is no evidence of a smal companion star. Any suggestions? Thanks, Alphons Franssen

    • Alphons,
      What instrument/camera/ISO did you use to make the photo? Rigel’s companion is very close, so you need fairly high magnification and good seeing to separate and see both stars clearly. That said, I think your exposure time may have been too short. .004 seconds is 1/250 of a second. I doubt you’d record the 7th magnitude companion with such a short exposure.

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