Supergiants?

Factual Questions
1

Okay, I know that “giant stars” result from extremely hot interiors driving the “outer envelope” of the star to a wide, diffuse range, resulting in a star with rather enormous effective diameter. And I know that most giant stars (in this sense) are red – low surface temperature.

I also know that the top of the Hertzsprung-Russell diagram holds supergiants all the way across, from coolest to hottest, and that supergiants are “even more diffuse and even greater in diameter than giants.”

But is there any real distinction between them other than an arbitrary point at which “this is diffuse enough and has a large enough diameter to be considered a supergiant”? And what’s the story behind blue-white (hot) supergiants? Do they really exist? How can they?

Bad Astronomer, where are you?

2

Well, Andre the Giant was a super star of wrestling… does that count?

3

Yeah, but Andre was of expatriate-Russian and French extraction, so neither ethnically nor politically was he a red giant. (Though when he exerted himself, he did tend to get a bit flushed…) :slight_smile:

Any serious answers?

4

Once upon a time, before I knew what a GIS was, I was an Astronomy major. While I’m not sure how blue-white SGs work (how long could those things last, anyhow?) I did find this about red SGs; apparently when the triple-alpha stage starts up is the dividing line (in this case, anyhow).

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redsup.html

5

From looking at the HR diagram, there certainly does appear to be a discrete difference between Giants and Supergiants. It doesn’t look like a nice bell curve to me. The Giants are all clustered together. There are definitely discreet masses which affect the end state of a star (White Dwarf, Neutron Star, Black Hole) so I’d venture to say there is something like this at work here as well. I have a hard time believing that the difference between Giants and Supergiants is related to mass though - I’d venture to say that the difference is related to the chemical composition of the original star - perhaps due to the amount of Helium.

6

a) I’ll look into it further, but off the top of my head, I want to say Vega, in Lyra the Lyre and Sirius in Canis Major are Blue stars…but are they giants? Hmm… I will check my ref’s tonight…

b) Also, a word about the Hertzsprung-Russell diagram: Please realize that most stars follow the “Main Sequence” as shown on the HR diagram - the typical life of a star. As for the blue giants, I’d WAG they’re not in the main sequence. Blue stars are (typ.) young stars. And, if they’re giants, they may not follow the “Main Sequence”.

Will get back to ya on this!

  • Jinx
7

Stars on the main sequence are placidly burning hydrogen into helium in their cores. When they run out of hydrogen in the core, the core begins to collapse.

In low-mass stars like the Sun, hydrogen burns for a while in a shell around the core, causing the star to swell into a poofy, cool red giant. Eventually the helium in the star’s core will begin to fuse into carbon. The star undergoes many fluctuations in core temperature, eventually blowing off its outer layers, which form a planetary nebula, leaving behind a white dwarf. As this happens, the star goes through a number of shenanigans in red-giant land, but does not become a supergiant.

Now, higher mass stars have enough mass that they can create more heat and pressure in their cores, allowing them to fuse hydrogen much more efficiently, so they rip through the hydrogen in their cores more quickly and evolve off the main sequence faster. Again, because of the higher mass, it can burn heavier elements in its core, so iron, not carbon, is the end of the line. The energy from this fusion can cause the star to get much hotter. This, plus the star’s greater mass, allows these star to become giants, brigher than red giants, and actually getting hotter and bluer over time. (Again, many shenanigans as shells around the core grow hot enough to burn new elements.)

The very hottest stars can become supergiants, which also get hotter (and bluer) as they use up their nuclear fuel. This happens quickly, so at any given time there are only a smattering of stars in this region of the H-R diagram. (So you’re right to be leery about blue supergiants, Poly; they’re ripping through their fuel and they just don’t last very long.)

Once massive stars have used up the heaviest elements they are capable of fusing, their cores collapse spectacularly, with the outer layers of the star crushing the electrons of the core into the protons, creating a neutron star. The inward-crashing layers are heated to tremendous temperatures (and from an astronomer, that means somethin!), so hot they it can fuse the heaviest naturally-occurring elements, up to uranium. This causes an explosion, blowing off the remenants of the star and scattering all those heavy elements through space (which is where all the heavy elements that make up you and me and the planet Earth came from!)

So (as far as I know) the line between giants and subgiants is not sharp or defined by anything fundemental. In fact, there are many classes of giants and supergiants, classified according to their luminosity. The luminosity of the star depends largely on its mass.

Hopefully, the Bad Astronomer will be along shortly to correct any mistakes I made.

8

You misspelled “fundamental” in the last paragraph. :smiley:

9

**Polycarp asked: "…I also know that the top of the Hertzsprung-Russell diagram holds supergiants all the way across, from coolest to hottest, and that supergiants are "even more diffuse and even greater in diameter than giants.

But is there any real distinction between them other than an arbitrary point at which “this is diffuse enough and has a large enough diameter to be considered a supergiant”? And what’s the story behind blue-white (hot) supergiants? Do they really exist? How can they?"**

Jinx answers: I believe you’re missing one concept with the H-R Diagram! Let’s back up one step. Hertzsprung and Russell (H-R) compiled a graph first known as the “spectrum-luminosity diagram”. While it is better known as the H-R Diagram, the original name refers to the fact that it is indeed a graph of absolute stellar magnitude (luminosity) vs. temperature (spectral class). Upon making this chart, it was surprising to find that the stars are not evenly distributed, but rather they fall into definitive regimes – of which “supergiants”, “giants” and “the main sequence” were defined as three of these regimes on the H-R diagram.

Examining the H-R diagram, you will find that the distinction of a giant from a supergiant is only absolute magnitude. For example, looking across the top of the H-R diagram, we find Rigel, in Orion, is a blue supergiant of about 20,000 degrees Kelvin. Whereas Antares, in Scorpio, is a red supergiant of only 4000 degrees Kelvin. (For comparison, the sun [although residing in the “main sequence” regime] is 6000 K.) However, both Rigel and Antares have an absolute stellar magnitude of about –5 or about 10,000x the brightness of the sun!

What about size of a supergiant? The chart itself does not directly consider relative sizes, but it is a stellar characteristic intrinsic of the absolute magnitude. It may surprise you to know that Antares has a radius about 6x Rigel’s! (It is worth noting that even a smaller supergiant, such as Rigel, is still significantly larger than the largest giants.)

What about temp? Well, let’s consider Rigel, in Orion, and Sirius A, in Canis Major. These are both blue stars of about 20,000 K. However, Rigel is a supergiant and Sirius A is a blue star in the Main Sequence. The difference? Once again, it is the apparent magnitudes which places these two stars in two different regimes on the H-R diagram. Rigel is 100x brighter than Sirius A. And, Sirius A is 100x brighter than the sun.

{Note: Apparent stellar magnitude allows us to easily compare “apples to apples” by examining the brightnesses of all stars as if seen from a common distance from earth. }

Again, the Bad Astronomer may wish to comment, but I believe this should answer Polycarps’s question about the existence of blue supergiants…and the technical distinction between giants and supergiants. - Jinx

10

[QUOTE]
*Originally posted by Podkayne *
…Now, higher mass stars…can burn heavier elements in its core, so iron, not carbon, is the end of the line. The energy from this fusion can cause the star to get much hotter. This, plus the star’s greater mass, allows these star to become giants, brigher than red giants, and actually getting hotter and bluer over time…

Podkayne, I was always under the impression blue stars were young stars. But, are you saying an older star can go from a red supergiant to a blue supergiant? I thought the star’s fusion with heavier elements causes the star to cool. I thought a red supergiant would either shrink to a red dwarf (or some dwarf stage), become a neutron star (brown dwarf?), or totally collapse upon itself becoming a black hole.

I’ve always found the “pathways” of the death of a star tricky to understand. Maybe you can show me what pieces to the puzzle I’ve been missing! - Jinx

11

Ahhh, excellent point. Recall what was said about the main sequence. More massive stars burn through their fuel more quickly. That means the more massive stars spend a shorter time on the main sequence. So if you see massive main-sequence stars in a star cluster, then you know it’s a young cluster. The key piece of information I didn’t mention is that more massive main sequence stars are hotter/bluer. They’re also brighter (which is just another way of saying that they’re burning their fuel faster, no?) so their light will dominate a young cluster of stars. That’s why blue=young stars.

This rule holds for main sequences stars, a.k.a. luminosity class V (that’s a Roman numeral five) or dwarf stars, which can be distinguished from giants and supergiants by their brightness and, I think, though I’m not 100% sure and can’t find a cite, through some subtle spectroscopic differences.

(N.B. These dwarfs are proper stars. White dwarfs or brown dwarfs are something different!)

Fusion of elements lighter than iron releases energy, feeding the star. Elements heavier than iron aren’t fused in stars, only in supernova explosions.

Red dwarfs are very low-mass main sequence stars. Neutron stars and black holes can only be created under the tremendous pressures of a supernova explosion.

There’s no peaceful end for a massive star. When its core has been converted entirely to iron, it can no longer undergo energy-creating nuclear reactions. Without heat to support the weight of the star, it crashes down, compressing the core into a neutron star, or, for the most massive stars, causing its total collapse to create a black hole. (And, I just realized I didn’t mention black holes above. Mea culpa!)

Caution: brown dwarfs are something else entirely. These are “failed stars” that never begin to fuse hydrogen. (Some can fuse tritium and deuterium.) The line between “big planet” and “brown dwarf” is more than a little blurry.

This is a really tough subject–and it doesn’t help that the terminology is so sloppy, with all those different dwarfs. I always have to go back to my intro astronomy book to remind myself how it all works. The discussion in my grad-level astrophysics book is too complicated to page through and find quick answers, though I have sworn to myself that I will work through those chapters again someday.

Now, I’m going to go practice writing “fundamental” one hundred times. :slight_smile:

12

Hey, Podkayne! When you’re not busy writing “fundemental”, can you give me some titles of your grad-level astrophysics books? I’d like to read more about it…maybe you have one title of a book that does an exceptional job on this topic?

Thanks! - Jinx

13

I wish someone could take the H-R diagram and make a flow chart to map out the possible pathways a star’s life may take - from birth to death. Is such a chart even possible?

  • Jinx
14

Thank you so much, Podkayne and all.

Lessee if I’ve got this right:

Red Dwarf: Low-magnitude, low-temperature, low-mass star at the bottom right of the Main Sequence.

White Dwarf: Degenerate-matter star at bottom of H-R diagram, below Main Sequence.

Brown Dwarf: Extremely-low-mass object, equivalent to “superjovian planet” which never got hot enough to institute core fusion. “Star” by courtesy, like a hasbeen actor charitably given a minor role in a blockbuster movie.

Green Dwarf: Refugee from a fantasy novel with no business in this thread.

Red Giant: High-mass star with helium fusion occurring in core, very diffuse and large of diameter and above main sequence.

Blue Giant: High-temperature, high-mass star at upper left of Main Sequence.

Supergiant: Very-high-mass star at top of H-R diagram. May be on extreme top of main sequence or above it. No clear delineation from giants. Depending on original mass, may be at any point horizontally (and hence any color and temperature).

Does that cover it?

Now, what about Wolf-Rayet stars? Ultra-hot, at extreme left of H-R table? How high on it are they? And what causes them?

15

No, RGs are actually low mass stars, from about 0.8 or so to 8 solar masses. The Sun will become a RG eventually. Past about that mass limit things are different, and you get a red or blue supergiant.

This is complicated, and to be honest I don’t have hard and fast answers at my fingertips. There are lots of webpages about this topic though. Try Astronomy Notes, for example. Also, the Astronomy Picture of the Day has many links to topics in astronomy; you can search on the word “evolution”, too.

16

With BA’s correction, Poly, looks like you’ve pretty much got it.

I should stress that even if there’s no sharp dividing line, supergiants are much rarer than giants (because they are only formed by the most massive stars, and they aren’t around for long), so if you just plot some population of stars on the H-R diagram, you see a densely-populated giant region, with a few supergiants scattered above them. The supergiants move around quite a bit in the upper-right of the H-R diagram (some even straying toward the hotter left side,) so it’s not just mass that determines where you’ll find them–the mass does place an upper limit on their luminosity, and constrains their temperature, ultimately.

Wolf-Rayet stars are typified by their strong stellar winds. They are blowing away their outer envelopes because their interiors are so hot. The most massive stars evolve from red supergiants into the Wolf-Rayet stage just before going supernova.

Jinx, here are pages with H-R diagrams showing the evolution of low-mass and massive stars.

An Introduction to Modern Astrophysics by Carroll and Ostlie is the text I studied for my qualifying exam, and the one I keep at my fingertips for reference on general questions. I don’t have anything else on stellar evolution, 'cause I just study rocks. :wink: