Presentation: medium-mass stars

Last time you were asked to ponder which car can drive farthest on a full tank: a Hummer H2, or a SmartCar fortwo? Perhaps stars are like cars...or maybe not.

In this presentation, we'll discuss the life, and more interestingly, the death of medium-mass stars.

First, main-sequence lifetimes of medium-mass stars, which is how long until they have depleted the hydrogen in their cores, fusing it into helium.

Consider the Hummer versus SmartCar range question. It turns out that they have the same range on a full fuel tank--but how can this be possible? The Hummer, with lower mileage than the SmartCar, would need to have a larger fuel tank. (This comparison uses 2008 data, as the Hummer H2 was discontinued soon afterwards.)

Now consider the main-sequence lifetimes of a medium-mass "sun-like" star and a low-mass "red dwarf"--calling to mind the previous Hummer versus SmartCar comparison of how long they can travel before running of fuel. As it turns out, stars are not like cars. A medium-mass star will run out of hydrogen in its core in 10 billion years, but the low-mass star will take much longer to run out of hydrogen to fuse--56 billion years, which is longer than the current age of the universe (14 billion years). So these low-mass red dwarfs are still chugging along, as none have yet to run out of fuel. How is this possible?

Which star fuses hydrogen faster: medium-mass or low-mass? How do you know this? Then in order for the low-mass star to "last longer," which star has a larger "fuel tank" capacity: medium-mass or low-mass? Does this make sense to you? Let's make this make sense.

If we look at cross-sections of these stars, we can see the convection currents that lie just under their surface photospheres, and the cores where energy is produced, due to high pressures and temperatures required for fusion. For the medium-mass star, once all the hydrogen in its core is depleted, its main-sequence lifetime is over. However, for the low-mass star, the convection currents just under its surface actually circulate down into the core, so as it depletes the hydrogen in its much smaller core, more hydrogen is circulated back in, so its "fuel tank" turns out to be the entirety of the low-mass stars--much larger than just the core of the medium-mass star. Imagine a SmartCar not only with better mileage than a Hummer, but with a fuel tank that was much larger than the Hummer's as well!

So stars are definitely not like cars...

How many stars will you find buried here? While all low-mass stars will eventually die, due to their extremely frugal use of large stores of hydrogen, none have ever died yet. So for the purposes of our discussion we will not worry about "Little Star Cemetery," but instead focus on the stars that will and have died: the medium-mass stars. (The deaths of massive stars will be covered in the next presentation.)

Second, how medium-mass stars die...alone.

Do you live alone, with no roommates? Does your refrigerator look like this? So what's for dinner tonight? First, probably the leftovers or frozen dinners--the easiest and most convenient things to eat. After that, what's left? If you must, you could always pull stuff out of your refrigerator to cook something nutritious and delicious. But once that stuff runs out, and you're still starving, then you might start getting desperate enough to eat the less delicious and nutritious stuff like mayonnaise, ketchup, and those wilted celery stalks from the bottom bin. Well, maybe you wouldn't go that far down the refrigerator food chain...

As it turns out, a medium-mass star has the same problem at the end of it main-sequence lifetime, having converted all the hydrogen in the core into helium. So what's left to eat for this star? Hydrogen is the convenient and most nutritious stuff for a star to eat. To fuse helium requires higher pressures and temperatures than hydrogen, and not as much energy would be released afterwards, so helium is not as convenient and nutritious as hydrogen, but hey, a star's gotta eat. But after fusing all the helium in its core into carbon, then that's where a medium-mass star draws the line, and it begins to die.

(A massive star, as we'll discuss in the subsequent presentation, has no qualms about going further down the refrigerator food chain, eating everything inside, even down to the takeout soy sauce packets and the brine left in bottom of pickle jars. But it's just putting off the inevitable, as once its refrigerator's has been emptied of anything of (debatable) nutritional value, it too will begin to die...)

So once a medium-mass star has reached the end of its main-sequence lifetime, depleting its hydrogen, it begins to take desperate measures to stave off "starvation," becoming a giant. Once its has converted all of its helium into much less tasty carbon, it will truly begin to die, and its outer layers will expand and dissipate as a planetary nebula, while the remaining core shrinks into a white dwarf. Like the desperate refrigerator scenario, let's introduce further analogies to extend our understanding of these giant, planetary nebula, and white dwarf stages beyond reading the textbook.

Since the less convenient and nutritious helium requires higher pressures and temperatures than hydrogen in order to fuse into carbon, the red/yellow giant stages are a result of a medium-mass star undergoing changes within to "jump start" helium fusion. Once helium fusion begins in the core, then things will keep chugging along until all the helium is converted into carbon.

While this happening, the outer layers of the medium-mass star will expand and cool. You can try this for yourself by "huffing" body-temperature breath from your lips onto the back of your hand. But if you were to purse you lips and "blow" this same body-temperature breath such that the air expands, it will feel cooler on the back of your hand...because it really is cooler.

As a medium-mass star depletes the helium in its core, the outer layers keep expanding and will dissipate outwards, much like a dandelion puffball.

The core will collapse and become a white-hot super-dense ball of carbon...and stay that way as it very slowly cools off. White dwarfs are boring, but hey, that's what you get for dying alone.

Third, what happens if you don't die alone (you get to take somebody with you).

Here two stars were born at the same time, but due to their different masses, have evolved at different rates (as discussed in the previous presentation) such the first star to die becomes a white dwarf, and will begin to pull hydrogen in from its companion star. Recall that a white dwarf is the remnant of a medium-mass star that underwent "star-vation" and could not go further past helium fusion. However, with a fresh supply of delicious, nutritious hydrogen that is easy to fuse, things get interesting.

If the white dwarf steals hydrogen from its companion star relatively slowly, then it will steadily build up a thin coat of hydrogen, and "flash-fuse" it, resulting in a nova explosion. The companion star is still contributing hydrogen, so in tens to hundreds of years, the white dwarf will build up another hydrogen coat to fuse, so these flashes would typically repeat at regular intervals. (Video link: "Explosions—Large and Small (Z Camelopardalis).")

If the white dwarf steals hydrogen from its companion star relatively quickly, then it will rapidly be smothered by a thick coat of hydrogen. This will drastically increase pressures and temperatures throughout the white dwarf, such that carbon fusion can finally begin, and as a result the entire white dwarf will undergo a type Ia supernova explosion. Here nothing is left of the white dwarf (and the surviving companion star as well). (Video link: "Artist's impression of vampire star.")

In order to consolidate the many details of medium-mass star death, let's have a picto-quiz, with figurative or actual representations of the stages a star like our sun will go through.

What stage is this? What did you notice that tells you this? (Well, this should be obvious.)

What stages are these? What did you notice that tells you this?

What stage is this? What did you notice that tells you this? What must be located in the center? (Which planet(s) would this resemble, as seen through an early telescope? Hence, the name.)

What stage is this? What did you notice that tells you this? What must be located in the center? (What do you think causes the "pinched" effect in the middle?)

What stage is this? What did you notice that tells you this?

Before this simulation movie clip begins, what is being shown here? What two possible stages might result due to these circumstances? (After watching the aftermath of this explosion, was your guess correct?) (Video link: "Animation of Tycho's....")