Cepheid variable stars are helping astronomers see what our galaxy looks like from within.
How many spiral arms does the Milky Way have? How far does its disk extend? Even the simplest questions about the galaxy we call home are still up for debate. Now, in what is likely only the beginning of a larger effort, three independent studies are using a famous class of pulsating star to map out hidden reaches of the Milky Way.
Classical Cepheid variables are aging massive stars whose fame derives from their bid to avoid gravitational collapse. Having run out of hydrogen to fuse in their cores, they struggle to burn shells of helium instead, puffing up and deflating at a rate directly tied to their intrinsic luminosity.
Now astronomers are increasingly studying Cepheids in near-infrared light, and they are beginning to pierce the veils of dust and gas that enshroud the Milky Way.
“Our knowledge of galactic structure is rather unsatisfactory . . . no consensus exists,” says Daniel Majaess (Saint Mary’s University and Mount Saint Vincent University, Canada).
That’s what motivates surveys such as VISTA Variables in Vía Láctea (VVV). The VISTA telescope at Paranal Observatory in Chile has already cataloged about a billion point sources. Since 2010, it has returned again and again to look at their variability over time. The VVV survey is expected to turn up hundreds of classical Cepheids — an incredible number when you consider the importance of just two Cepheids on the far side of the Milky Way.
István Dékány (Millenium Institute of Astrophysics, Chile, and Pontificia Universidad Católica de Chile), Majaess, and colleagues announced the discovery of two Cepheids 37,000 light-years from Earth and 11,000 light-years from our galaxy’s center. The pair is remarkably close, separated by only 3 light-years, and both are between 45 million and 51 million years old.
Given their similarities, these Cepheids were most likely born in the same star cluster in the “Far 3 kpc Arm,” a spiral arm thought to circle the far side of the galaxy’s star-packed bulge. But deeper observations will need to confirm this — the intense amount of dust and the sheer number of stars in the bulge prevent the authors from identifying any other members of the alleged cluster. For now, they dub it the “Invisible Cluster.”
Just two weeks after Dékány’s team published their results, another study (now accepted to Astrophysical Journal Letters) reported an additional four Cepheids in VVV data. But the pulsing stars found by Sukanya Chakrabarti (Rochester Institute of Technology) and colleagues were nowhere near the bulge — they appear to be almost 300,000 light-years from the Sun.
You read that right. These stars lie far beyond the Milky Way’s disk, which is about 100,000 light-years across. And this find is all the more interesting because Chakrabarti had predicted the existence of a dwarf galaxy at that exact location six years ago.
Astronomers had previously detected ripples in the outer part of the Milky Way’s gas disk, which can’t be explained in an isolated spiral galaxy. Instead, such ripples might come from gravitational interactions with smaller, dwarf galaxies. Chakrabarti ran simulations that showed a dwarf one-hundredth the mass of the Milky Way could have passed through our galaxy to create the mysterious ripples. The dwarf would have escaped detection until now because it’s dim and lies right behind the Milky Wayplane from our perspective. She even predicted its orbit and structure.
“I had hoped that because the prediction was very specific, observers would try and search for this dwarf galaxy,” Chakrabarti adds. “I finally decided to look for it myself.”
And she seems to have found exactly what she was looking for. But there’s a catch.
The classical Cepheids researchers tend to look for are the more common variable stars: massive, young (even as they near the end of their lives), and pulsating. But a second type of Cepheid variable, called Type II, are actually much less massive stars (half the mass of the Sun or so) and about 100 times older, so their intrinsic luminosity is much fainter.
Trouble is, Type II Cepheid light curves masquerade as classical Cepheids pretty well, with the same general shape of pulsations. And if Chakrabarti’s Cepheids are of the Type II variety, then they lie only 160,000 light-years away. They’d still be an interesting (and puzzling) find, but one that wouldn’t match the precise prediction for the dwarf galaxy.
But Chakrabarti’s money is on these Cepheids being of the classical variety. For one, Type II Cepheids are less common, and therefore less likely. And Chakrabarti and colleagues also see red clump stars, luminous giant stars that in aggregate serve as another kind of standard candle, at the same location in the sky and at the same distance.
Spectroscopy will cinch the matter, though it will be a challenge for these faint stars. If the team can get radial velocity observations, they can see whether the clump of Cepheids moves together, and if they move in the way predicted for the dwarf’s passage through the Milky Way.
Still More Cepheids
All of these results come after a study published last year in the May 15th Nature. Michael Feast (University of Cape Town, South Africa) and colleagues followed up on 32 possible Cepheids discovered in the Optical Gravitational Lensing Experiment (OGLE), using the South African Large Telescope (SALT) and the Infrared Survey Facility. The spectroscopy and near-infrared images allowed the team to more definitely determine the “classical” status of five Cepheids.
These five Cepheids again lie in the outer reaches of the Milky Way’s disk, though at distances less extreme than those found in Chakrabarti’s study, between 42,000 and 72,000 light-years from the galaxy’s center.
Since classical Cepheids are massive and young, they are typically associated with spiral arms. It would be odd to see one of these stars sitting out in the galactic halo. So Feast’s team suggested that these Cepheids lie in the Outer Scutum-Centaurus arm, which would have to flare to account for the Cepheids’ spread above and below the galactic plane.
“Clearly, these Cepheids are just the tip of the iceberg,” Feast and colleagues write. The VVV survey and others are well on their way to revealing the rest.
Sukanya Chakrabarti et al. "Clustered Cepheid Variables 90 Kiloparsecs from the Galactic Center." Accepted for publication in Astrophysical Journal Letters.
István Dékány et al. "Discovery of a Pair of Classical Cepheids in an Invisible Cluster Beyond the Galactic Bulge." Astrophysical Journal Letters, 19 January 2015.
Michael Feast et al. "Cepheid Variable in the Flared Outer Disk of Our Galaxy." Nature, 15 May 2014.
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