The meetings of the American Astronomical Society never disappoint lovers of astronomy and the Universe, since many scientists take advantage of them to present their latest findings. And the meeting that is being held these days in Seattle (Washington), number 241, has not been an exception to this rule.
Two talks, the first on January 9 and the second today, the 11th, this time have taken much of the limelight, because in them Yuting Feng and Guha Thakurta, astronomers from the University of California at Santa Cruz, have announced the discovery of the most distant stars in our galaxy. About 200 distant variable starsknown as RR Lyrae, which lie more than a million light-years from us, at the outer edges of the Milky Way’s halo. Something really incredible, given the size of our galaxy, about 100,000 light years in diameter, and which is almost equivalent to half the distance that separates us from Andromedaour neighboring galaxy 2.5 million light-years away.
The characteristic pulsations and brightness of RR Lyrae stars make them excellent ‘standard candles’ for measuring galactic distances. So these new observations allowed Feng and Thakurta to discover how far the Milky Way’s halo really extends. ‘This study,’ says Thakurta, ‘is redefining what constitutes the outer limits of our galaxy. The Milky Way and Andromeda are so large that there is hardly any space between the two galaxies.”
During his talk, Thakurta explained that our galaxy’s stellar halo is much larger than the disk, which is about 100,000 light-years across. Our Solar System resides in one of the spiral arms of the disk. In the middle of the disk is a central bulge, and surrounding it all is the halo, containing the oldest stars in the galaxy and stretching for hundreds of thousands of light-years in all directions. “The halo – lavishes the astronomer – is the most difficult part to study because its outer limits are very far away. There the stars are very sparse compared to the high stellar densities of the disk and bulge, but the halo is dominated by dark matter and actually contains most of the galaxy’s mass.”
A confirmed theory
Feng, for his part, points out that previous studies had already calculated that the stellar halo should extend around 300 kiloparsecs (or 1 million light-years) from the galactic center. (A kiloparsec is equal to 3,260 light years.) And now, those distances are confirmed, since the 208 RR Lyrae stars detected by Feng and his colleagues were between 20 and 320 kiloparsecs away. “We were able to use these variable stars as reliable trackers to pin down distances,” says Feng. Our observations confirm theoretical estimates of the size of the halo, so this is an important result.”
“The way these stars vary their brightness,” explains Thakurta, “is similar to an electrocardiogram. They’re like the heartbeat of the galaxy, so the brightness increases quickly and decreases slowly, and the cycle repeats itself perfectly with this very characteristic shape. Also, if you measure their average brightness, it turns out to be the same from star to star. And this combination is fantastic for studying the structure of the galaxy.”
As astronomers know all too well, a star’s brightness can be misleading. Some are brighter than others, but a star can look bright because it’s really so bright or because it’s so close to us, and it can be hard to tell the difference. But astronomers can identify an RR Lyrae star from its characteristic pulsations, and then use its observed brightness to calculate how far away it is. The procedures are not simple, and sometimes the most distant objects, such as quasars, can masquerade as RR Lyrae stars.
“Only astronomers know how painful it is to get reliable markers of these distances,” Feng said. And now this robust sample of distant RR Lyrae stars gives us a very powerful tool to study the halo and test our current models of the size and mass of our galaxy.”