It's Real. We Found a Green Star.（原声文稿）

Transcript of 绿色恒星找到了：为什么你却几乎看不到 （中文完整文稿）。

Stars come in a handful of colours. The coolest glow red and orange.

Our sun looks white, because our atmosphere scatters some of its blue light away, it looks yellow from the ground.

Meanwhile, the hottest stars burn blue.

But you will never see a green star. Physics simply doesn't allow it. Or so we thought.

So, when astronomers recently announced that they had found a green star, it caught everyone by surprise.

And the reason it's green is even stranger than the colour itself.

This star is green because of what it did to its own planets.

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The green star is a white dwarf.

A white dwarf is what's left behind when a star, like our sun, dies.

Near the end of its life, the star swells into a red giant, hundreds of times larger than before.

Then it gently sheds its outer layers into space, and all that remains is its core.

That's exactly what will happen to our own sun.

In about 5 billion years it will run out of hydrogen at its core and start to swell. shell.

As a red giant, it will grow so large that it swallows Mercury and Venus, and possibly Earth.

Even if Earth survives, it will be a scorched rock long before then, because the Sun is slowly getting brighter, and will boil away our oceans in about a billion years.

Then the Sun will shed its outer layers, creating a glowing shell of gas called a planetary nebula, and leave behind a white dwarf about the size of Earth.

With no fuel left to burn, it will spend billions of years slowly cooling and fading in the dark. And it will be incredibly dense.

A typical white dwarf packs more than half the mass of our sun into that tiny space. A teaspoon of it would weigh several tons.

Now, there are billions of white dwarfs in our galaxy.

One of the most famous orbits, Sirius, the brightest star in our night sky.

It's so small and so close to its bright companion that it's almost impossible to spot without a powerful telescope.

In 1917, at Mount Wilson Observatory in California, astronomers photographed the light of a white dwarf called Van Maynen's star, just 14 light years from Earth.

At the time, astronomers didn't even know what a white dwarf really was.

It would be years before anyone understood just how dense these stars are.

When they split its light into a rainbow, they found dark lines in it.

Those lines are fingerprints, and they showed that the star's atmosphere contained calcium, magnesium and iron. That shouldn't have been possible.

The gravity of a white dwarf is so strong that heavy elements like these should sink deep inside it and vanish from view unless something was constantly adding fresh material to its surface. Nobody could explain it.

It wasn't until the 2010s that astronomers looked at that old photograph again and realized what they were seeing.

The calcium, magnesium and iron were the remains of rocky bodies shredded and swallowed by the star.

That photograph turned out to be the earliest evidence ever recorded of a planetary system around another star.

Astronomers now wanted to catch a white dwarf in the act and in 2015 they did.

NASA's Kepler space telescope was watching a white dwarf called W .D.

1145 plus 017, about 570 light -years away in the constellation Virgo.

Every 4 .5 hours, something passed in front of it and blocked up to 40 % of its light.

The dips in brightness didn't have the clean shape of a solid planet.

They were stretched out and lopsided, like the shadow of a comet with a long tail.

It was a rocky body, about the size of a large asteroid, circling the white dwarf at roughly twice the distance between Earth and the Moon.

The star's gravity was tearing it apart, and its dust was streaming away behind it.

When a star like our Sun becomes a red giant and then a white dwarf, its planetary system is thrown into chaos, leaving the surviving planets on new orbits.

and over millions of years their gravity can fling asteroids and comets inward straight toward the white dwarf.

When those rocky bodies get too close they're ripped apart, their dust rains down onto the star and the elements they were made of get mixed into its atmosphere.

That's exactly what happened to the 80 light -years away in the constellation Draco.

It's a cool white dwarf at around 4000 degrees Celsius and its atmosphere is loaded with sodium, calcium and lithium from the rocky material it has swallowed.

On a star like our sun, that wouldn't change much, but the atmosphere of a white dwarf is nothing like the sun's.

Its gravity crushes the gas so tightly that in the coolest With glist white dwarfs, it can become about as dense as water.

Packed that tightly, the sodium in the green star's atmosphere is constantly being slammed by helium atoms.

That makes it soak up a much wider range of light than it normally would.

It swallows the yellows, the oranges and some of the reds.

Whatever light is left over gives the star a pale yellow -green glow, a colour the team describes as chartreuse.

A star that should look orange -white ends up green because of the planets it destroyed. And it isn't the only white dwarf with a strange colour.

The same team studied 151 white dwarfs within about 65 light -years of Earth and worked out what colour each one would look like to the human eye.

Most of them were white, just as the name suggests, but a few were very different.

One of them, called WDJ1011 plus 2845, is about 48 light -years away.

It may be the reddest star ever recorded.

Deep inside it, carbon is being churned up from its core into its atmosphere, and that carbon gives it a soft, salmon -pink glow.

Other white dwarves with carbon in their atmospheres would look pink or even purple.

– some with a mix of hydrogen and helium would have a cyan tint.

And then there's WDJ 2356 -2054.

It's only about 4040 degrees Kelvin, which is cool enough that it should glow orange.

Instead, its thick atmosphere strips out so much of the yellow and red light that it would look a vivid blue.

It's a cool star dressed up as a All together, the team estimates that around 1 in 30 white dwarfs could show a colour nobody expected.

Now, there's one thing worth being clear about. No one has actually seen these colours.

White dwarfs are far too small and far too faint to see their colour from Earth.

Instead, astronomers took the light from each star, split it into a rainbow, and calculated calculated what colour a human eye would see if you could fly close enough to look at it directly.

So the green colour is calculated from the star's light.

It shows the colour you would see if you could travel 280 light years and look at it for yourself.

Because the swallowed material stays in the star's atmosphere, a white dwarf works a little like a forensics lab.

By measuring which elements are there and how much of each, astronomers can work out what the destroyed planets and asteroids were made of.

They can tell whether those bodies were rocky like earth or icy like a comet, and even whether they had metal cores.

So far, most of the destroyed bodies studied this way turn out to be rocky, built from many of the same elements as our own planet.

Some even look like pieces of a shattered world, chunks of a planet's crust or its iron core falling into a dead star one piece at a time.

And there are many more of these stars waiting to be studied.

Right now, astronomers have detailed light measurements for only a fraction of the white dwarfs they know about.

New sky surveys are about to change that, increasing the number of white dwarfs with this kind of data about ten -fold.

The lead researcher hopes that could take us from one or two known green white dwarfs to ten or twenty, which brings us back to our own sun.

One day, when it has become a white dwarf, whatever survives of our solar system will still be circling it, the leftover planets, asteroids and comets, and over time some of that debris will fall in, just like it has around these stars.

Billions of years from now, somewhere far away, another civilization might point a telescope at our sun, split it hits light into a rainbow and read the remains of our solar system, written across its surface.

They might even work out what colour it is. Isn't that fascinating?

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