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Megastar!

Astronomers get their first close look at what may be the brightest star in the galaxy. And they're totally dazzled by what they've see.

It blazes with the light of 10 million Suns. It spans at least 300 kilometers (186 million miles) in diameter--so huge it would stretch from the Sun to the Earth's orbit. It may be the brightest star ever seen in our galaxy, the Milky Way. Unbelievable? Some astronomers thought so too--until three months ago.

Last September the Hubble Space Telescope transmitted vivid photos from space of what could be the Milky Way's most luminous star. Astronomers have dubbed it "Pistol Star," for the gun shape of its nebula, or surrounding gaseous cloud. "What we found was just absolutely amazing!" says astronomer Don Figer, head of the investigating research team at the University of California at Los Angeles.

Yet for all its dazzle, you won't see the Pistol Star with your naked eye, or even through the most powerful telescopes on Earth. That's because vast clouds of intersteller dust block its stellar rays.

Just as celebrity photographers use a telephoto lens to snap Hollywood superstars, astronomers turned a new supercamera on the very venter of the Milky Way to catch the elusive Pistol Star. The camera, recently installed on the Hubble Space Telescope, is called the Near-Infrared Camera and Multi-Object Spectrometer. It detects heat emitted by objects in space, even those stars screened from view by interstellar clouds (see Hubble timeline, p. 10).

GALAXY CENTRAL

What may be most exciting about this megastar is its location--near the very core of the Milky Way. That means the Pistol Star is 25,000 light-years away from Earth. (A light-year is the distance a beam of light travels in one year, or 9346 trillion km/5.88 trillion mi.)

For a long time astronomers thought the galactic center was full of old or dying stars. With the Milky Way's core hidden from Earth's optical telescopes by such vast clouds, scientists had no way to disprove this theory.

But the advent of radio astronomy 30 years ago and infrared imaging technology 10 years ago made the clouds practically transparent. Now, instead of relying on a celestial object's visible light to see it, astronomers detect an object through its radio and heat waves.

Peering at the galactic center through Hubble's new infrared camera, Figer finds the center "an extremely exciting place." Colossal forces are at play at a black hole, right at the Milky Way's heart. The black hole isn't empty space--it's a dead star that collapsed inward on itself. The black hole's gravitational pull (attractive force) is so strong it swallows its own light. That pull makes it almost impossible for stars to form nearby--unless they're massive, Figer believes.

Until now, astronomers thought they knew how all stars--big or small--formed anywhere in the universe (see Life and Death of a Star, p. 8).

But the Pistol Star is causing astronomers to rethink how stars are born near the center of the Milky Way. Turbulent energies here drive particles into a speeding frenzy. That makes it harder for them to collide and stick together. Plus the black holes and other nearby stars' gravitational pull would shred any protostar to pieces.

But an extremely dense cloud might accumulate enough mass to form a protostar so huge it resists these forces, Figer speculates. "It's like, `Who cares--I'm just going to collapse," he says.

EXTREME STAR

No one knows for sure how stars become as massive or luminous as the Pistol Star. When a star ignites, it releases an immense amount of radiation. That outpouring of of radiation halts any matter from flowing in--the star reaches its maximum mass. Beyond this limit a star will theoretically blow itself apart.

But astronomers don't know what the limit is. Until they find even more massive stars, the Pistol Star is the max. The star measures 300 million to 450 million km (186 million to 280 million mi) across. It weighs about 100 times the mass of the Sun. And it may emit as much energy in six seconds as the Sun does in a year!

This enormous energy output translates into incredible shine--but not necessarily shine we can see on Earth. And that's why astronomers measures a star's brightness two ways: by its apparent magnitude (how bright a star looks when seen from Earth) and by its absolute magnitude (how bright a star would look if it were 33 light-years away from Earth). Oddly, the brighter a star is, the lower its magnitude numbers. The Pistol Star has an apparent magnitude of 35--it's not at all visible from Earth. But the star has an absolute magnitude of-12.5, making it the most dazzling star in the sky.

Despite its brightness, the Pistol Star is by no means the hottest star around. It's only about three times as hot as the Sun. Astronomers determine the Pistol Star's temperature by its blue-white color.

However, the Pistol Star will quickly burn itself out. Figer estimates the megastar will last only 1 to 3 million years longer before it explodes. The Pistol Star is already about 1 to 3 million years old. Our Sun, on the other hand, is 5 billion years old, and can probably last for another 5 billion years. Compared to the Pistol Star, it's a low-energy burner.

One big question that faces astronomers now is: Are massive stars typical of galactic centers?

"It would be worthwhile to look at the centers of other galaxies to see if conditions there favor the formation of very massive stars," says Michael Rich, an astronomer at Columbia University in New York City, and co-investigator of the Pistol Star. That's exactly what he and Figer plan to do.

They'll also determine if the Pistol Star is really single star, or a binary system--two stars orbiting each other closely. "We clearly have more work to do," Figer says. Stay tuned! Could this megastar have a sequel in the works?

RELATED ARTICLE: Life and Death of a Star

1 Inside a cloud of intersteller gas and dust, gravity pulls particles together to form clumps, called protostars.

2 As gravity causes a protostar to contract, the protostar's core grows hotter. When the core reaches 1,100,000 [degrees] C, nuclear fusion ignites the star.

3 Swirling gas and dust that surround the young star may eventually form into planets orbiting the star.

4 At the star's core, hydrogen atoms fuse to make helium atoms. This nuclear fusion releases energy in the form of heat and light.

5 After about 10 billion years, the star runs out of hydrogen fuel. The core collapse and the star's outer layer expands and cools. The star is now a red giant.

6 After a billion years, the red giant ejects its outer layer as a planetary nebula, leaving a collapsed core at the center.

7 As the outer layer disperses into space, the hot core, known as a white dwarft, is exposed.

8 For several more billion years, the white dwarf cools down into a dark globe. The star is dea.

RELATED ARTICLE: HUBBLE: A TELESCOPE'S TALE

The Hubble Space Telescope (above) keeps an eye peeled on the universe as it orbits 611 km (380 mi.) above the Earth's surface. Without our murky atmosphere to cloud the lenses of its three cameras, Hubble can snap shots that are sharper than any telescope's on Earth.

Since 1990, Hubble has taken hundreds of thousands of images. Besides capturing never-before-seen regions of space, the telescope helps scientists learn more about how stars and galaxies form, and how old the universe is. Here's the scoop on some of Hubble's ups and downs:

1977

work begins to design and build Hubble.

1990

Hubble is launched into orbit the space shuttle Discovery. But its first images are blurry! NASA realizes the curved mirror that's used to gather light for the telescope is too flat--by 1/50 of a human hair.

1993

Four astronauts latch on a refrigerator-sized box containing 5 pairs of small mirror "eye-glasses" to fix Hubble (above).

1994

Using Hubble, scientists view the surface of Pluto for the first time since the planet was discovered in 1930.

1994

Hubble spies "bruises" on Jupiter's surface from fragments of Comet Shoemaker-Levy 9 as the comet slams into the planet.

1995

Hubble looks on as a star is born in the Eagle Nebula. The tall Columns are gas and dust collapsing into a star.

1995

Hubble points its cameras at a spot in the universe smaller than a grain of sand--and sees more than 1,500 galaxies.

1997

Hubble is set up with more high-tech instruments, including the infrared camera that detects the "Pistol Star."

RELATED ARTICLE: Star Colors

A star's color indicates its surface temperature. Below are examples of stars in our galaxy.

Blue

Star: Rigel 50,000 [degrees] C (90,000 [degrees] F)

Blue-white

Star: Pistol Star 20,000 [degrees] C (36,000 [degrees] F)

White

Star: Sirius 10,000 [degrees] C (18,000 [degrees] C)

Yellow

Star: Sun 6,000 [degrees] C (10,800 [degrees] F)

Orange

Star: Aldebaran 4,500 [degrees] C (7,200 [degrees] F)

Orange-red

Star: Arcturus 3,500 [degrees] C (6,300 [degrees] F)

Red

Star: Betelgeuse 3,000 [degrees] C (5,400 [degrees] F)
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Article Details
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Title Annotation:includes related information on the history of the Hubble Space Telescope; astronomers study an extremely bright star called Piston Star
Author:Chang, Maria L.
Publication:Science World
Article Type:Brief Article
Date:Dec 8, 1997
Words:1546
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