04 Track 4

Unit 8

Beyond Planet Earth

Chapter 2

Star Struck

Page 141

Star Struck

It’s hard to be modest when you live in the Milky Way. Our galaxy is brighter and more massive than most other galaxies. From end to end, the disc-shaped Milky Way spans 120,000 light-years.

Encircling it as another disk, composed mostly of hydrogen gas. And enclosing that is an enormous halo of dark matter. Indeed, our galaxy is so huge that dozens of lesser galaxies circle it, like moons orbiting the planet.

Giant galaxies like the Milky Way and the nearby, even larger, Andromeda galaxy possess the power to create a rich supply of iron, oxygen, and other heavy elements (heavier than hydrogen and helium).

Such heavy elements are the building blocks of planets, and are essential for life. When a star explodes in the smaller galaxy, this raw material shoots out into space and is lost.

But in the vast Milky Way, the elements encounter gas and dust along their path, and are restrained by the strength of the galaxy’s gravitational field. These impediments slow their speed, so they can enrich star-forming gas clouds with the ingredients for new generations of stars and planets.

That’s what happened 4.6 billion years ago, when the Sun and Earth were born.

Because we reside within the Milky Way, we actually know less about its overall appearance then we do about distant galaxies.

Think about it this way: in the absence of a mirror, you know more about your friends’ faces than your own.

Nevertheless, in the past decade astronomers have made numerous startling discoveries about our galaxy, beginning with the revelation that a huge black hole lies at its heart.

Every star in the Milky Way revolves around this black hole, named Sagittarius A * (pronounced “Sagittarius A-star”).

The Sun, 27,000 light-years away, completes a revolution once every 230 million years.

Within just a light-year of the black hole, there are more than 100,000 other stars caught far more firmly in its grip. Some take only a few years to complete their orbits.

These paths reveal that Sagittarius A * is four million times the mass of the sun.

Every now and then, the black hole swallows a planet or star like a particularly voracious vacuum cleaner. The victim is heated to such high temperatures that it lets out an explosion of x-rays which light up nearby gas clouds.

This helps keep a record of the black hole’s past feasts. For example, in 2004, scientists reported an x-ray echo in the gas cloud some 350-light from the black hole. Since x-rays travel at the speed of light, the echo indicates that an object fell into the black hole around 350 years ago.

The x-ray intensity suggests it had the mass of a small planet. Surprisingly, the black hole also flings stars away. In 2005 astronomers reported a star racing away from the galactic center at 709 kilometers a second.

At that speed, it will soon leave the galaxy. fifteen more of these fast-moving stars have since been discovered. Over the Milky Way’s lifetime, the black hole may have flung a million stars out of the galaxy.

Despite the violence around the black hole, the galactic core is a fertile place. Stars congregate at the galaxy’s center, so the life-giving heavy elements they create are most plentiful there.

Even the newborn stars near our sun − halfway between the black hole and the edge of the galaxy possess orbiting discs of gas and dust that survive long enough to give birth to planets.

In contrast, prospects for planets at the galaxy’s edge are bleak. Over a hundred newborn stars at the edge of the Milky Way were recently found to have low supplies of heavy elements.

For example, their oxygen content was only 20 percent of the sun’s. Most of these young stars have already lost their planet-forming discs of gas and dust. No discs, no planets; no planets, no life.

Stars with even lower amounts of oxygen and iron offer insight into the birth of the galaxy itself. Residing in the halo extending above and below the galaxy’s disc, these stars are so ancient that they formed before earlier generations of stars had a chance to produce heavy elements.

Astronomers date these stars, and hence the age of the entire galaxy, by studying globular clusters, tightly packed groupings of older stars.

However, estimates of their ages depend on theories of how stars live and die. Fortunately, there is another way to measure the galaxy’s age. In 2005, a German graduate student named Anna Frebel began looking for individual stars in the halo.

She discovered one that was chemically primitive − it had relatively few elements, meaning it must have been born before many generations of stars had chemically enriched our galaxy.

And this halo star contained elements that are far heavier than iron, including uranium. She measured the amount of uranium using a method similar to carbon dating, and estimated the star’s age at around 13.2 billion years old.

That figure suggests that the Milky Way is only slightly younger than the universe itself, which is 13.7 billion years old. The mighty galaxy, whose countless stars would later make life possible on Earth, certainly didn’t waste any time being born.