11 January 2015
Astronomers from Chile and Canada have recently detected the first pair of a type of pulsating star called Cepheids. to be found on the far side of the Milky Way.
This Hubble image shows RS Puppis, a type of variable star known as a Cepheid variable. As variable stars go, Cepheids have comparatively long periods— RS Puppis, for example, varies in brightness by almost a factor of five every 40 or so days. RS Puppis is unusual; this variable star is shrouded by thick, dark clouds of dust enabling a phenomenon known as a light echo to be shown with stunning clarity. These Hubble observations show the ethereal object embedded in its dusty environment, set against a dark sky filled with background galaxies.
It seems that they may be part of a young, open star cluster, making this a potential double-discovery win for the team as neither object has been discovered on the opposite side of the Galactic disc to the Earth before.
By using infrared data from a survey called the Vista Variables in the Via Láctea (VVV), using the VISTA telescope based at Paranal Observatory, Chile, the team created a new observing programme, called the VVV Galactic Cepheid Program (VGCP). Its aim was to discover and characterise distant Cepheids with the aim of mapping the spiral arm structure on the far side of the Milky Way’s disc, behind the central bulge.
As part of their first results from VGCP, the team were able to detect a close pair of classical Cepheids that are almost identical in their observed properties. These “Twin Cepheids” both have the same apparent brightness and colour (temperature), are located at the same distance away from the Sun, and are obscured by the same amount of dust from the central bulge.
From these similarities and subsequent models, it is believed that both stars are the same age at around 48 million years, have a separation of just over 3 light-years, and are located more than 37,000 light-years away.
False colour VVV image taken from the paper, showing the location of the two Cepheids highlighted by the white marks. The image scale given is 30 arcseconds, or 0.11 light years. Image credit: I. Dékány et al. 2014
Cepheid variables are very large, luminous yellow stars in the later stages of stellar evolution that expand and contract which causes their brightness to vary over a period of time between one and 70 days. They are therefore classified as “pulsating variable stars”.
The name Cepheid originates from the first star of its type ever discovered in 1794, Delta Cephei in the constellation Cepheus. Delta Cephei is one of four Cepheids visible to the naked eye from Earth. These stars are commonly referred to as a “standard candle” of distance measurement, as they follow a very reliable “period-luminosity relationship”.
The period over which the star completes one pulsation tells us how bright it must really be intrinsically. Comparing that to the star’s apparent brightness in the sky allows its distance to be gauged precisely.
This feature made Cepheids very important in helping astronomers to map the size and structure of the Milky Way, in particular its spiral arms. And their detection in nearby galaxies helped convince astronomers that these really were collections of stars outisde our own Milky Way.
Since classical Cepheids are relatively rare, the chance of finding two so similar and so close together is practically zero. “Finding such Twin Cepheids is quite as unlikely as winning the lottery!” said Dr István Dékány, lead author on a paper accepted for publication in Astrophysical Journal Letters.
It is believed that the Twin Cepheids may be part of a larger “open cluster“, which is a grouping of a few thousand stars of similar age that formed from the same initial molecular cloud. Currently, the presence of the open cluster is only inferred as the region obscured by the dense dust and stars of the Milky Way’s central bulge which makes it difficult to observe them. As a result, the team hope to acquire deeper observations in the future to confirm its existence.
“Our current, popular face-on maps of the Milky Way are analogous to early maps of the globe from the 16th century, where most of the continents were already depicted but distances and shapes were totally distorted,” explains Dékány.
There is a whole other, as-yet relatively unknown locale on the opposite side of the Galaxy, buried within the gas and dust, that will keep the team busy for a while. “What we see with the latest technology is still only the tip of the iceberg,” Dékány concludes.
See the full article here.
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