Science

Hubble telescope takes photos of ancient white dwarfs at Milky Way's centre

NASA's Hubble Space Telescope has for the first time observed a cluster of white dwarfs — the tiny, super-dense remains of burned-out stars — at the centre of the Milky Way.

1 teaspoon of matter from a white dwarf would weigh about 14 tonnes

A ground-based view of the Milky Way’s central bulge, seen in the direction of the constellation Sagittarius. Giant dust clouds block most of the starlight coming from the galactic centre. Hubble however, peered through a region (marked by the arrow) called the Sagittarius Window, a keyhole view into the galaxy’s hub. (A. Fujii/NASA)

NASA's Hubble Space Telescope has for the first time observed a cluster of white dwarfs — the tiny, super-dense remains of burned-out stars — at the centre of the Milky Way.

The telescope's "cosmic archeological dig," as NASA calls it, is the most detailed look at the central bulge of the galaxy. Scientists believe it's the oldest part of the Milky Way. Many of the white dwarfs currently observed once shone brightly about 12 billion years ago.

"It is important to observe the Milky Way's bulge because it is the only bulge we can study in detail," said Annalisa Calamida of the Space Telescope Science Institute (STScI) in Baltimore.

"The Milky Way's bulge includes almost a quarter of the galaxy's stellar mass. Characterizing the properties of the bulge stars can then provide important information to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies."

Hubble uncovered extremely faint and hot white dwarfs. This is a sample of four out of the 70 brightest white dwarfs spied by Hubble in the Milky Way's bulge. (NASA/ESA/STScI/SWEEPS Science Team)

26,000 light-years away

The region Hubble mapped, part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS), is located 26,000 light-years away. The team focused its research the hottest observable white dwarfs, about 70 in total.

"These 70 white dwarfs represent the peak of the iceberg," Sahu said, adding that more powerful telescopes in the future will be able to count even the faintest stars in the Milky Way's bulge.

"We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge. 

A small section of Hubble's view of the dense collection of stars crammed together in the galactic bulge. The region surveyed is 26,000 light-years away. (NASA/ESA/STScI/SWEEPS Science Team)

Formation of white dwarfs

White dwarfs are the burned-out remnants of stars once about eight times the mass of our sun. After burning off all of their hydrogen during their "main sequence," a star will cool down, expand and become a red giant. (When its main sequence ends, the sun will expand to a red giant about 200 times its current diameter, incinerating Mercury, Venus and possibly Earth.)

Some red giants eventually blow out most of their mass, leaving behind a tiny compressed core known as a white dwarf about the size of Earth. 

They are incredibly dense: a teaspoon of a white dwarf's matter would weigh about 14 tonnes. Only black holes and neutron stars are denser.

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