The Polaris Cluster

May 28, 2010
A Chandra X-ray Observatory image of the stars around Polaris (identified with an A). The other labels mark the positions of stars seen in the visible and thought to be physically associated with Polaris A.

(PhysOrg.com) -- A Cepheid star is one whose mass and age results in physical conditions that generate periodic oscillations in its photosphere. A Cepheid thus varies regularly in brightness, with a period proportional to its intrinsic luminosity.

This extraordinarily useful property of Cepheid variables, discovered and calibrated at Harvard by Henrietta Leavitt in 1908, allows them to act as reliable cosmic distance calibrators. By comparing the intrinsic brightness as determined from a period (which is easily measured) with the measured brightness, a precise distance can in principle be obtained. Cepheids in distant that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the expanding universe model (the "big bang" model).

The North Star, Polaris, is not only renowned as a reliable beacon for early navigators. It is also the closest Cepheid to earth (about 425 light-years away), and a subject of intense study. One issue is whether, like many stars, it is associated with a cluster of small companion stars that could have affected its evolution. In fact Polaris itself ("Polaris Aa", whose mass is 4.5 solar-masses) is known to orbit with a close companion, Polaris Ab (whose mass of 1.3 solar-masses).

The pair orbit at a separation of about 15 astronomical units, about as far apart as Uranus is from the sun. Another nearby star, Polaris B, seems to be orbiting around the other two at a distance 100 times farther away. Two more stars nearby, Polaris C and D, might also be faint companions that some astronomers think are gravitationally bound to the others.

Four CfA astronomers, Nancy Evans, Scott Wolk, Margarita Karovska, and Bradley Spitzbart, together with four colleagues, used the Chandra X-ray Observatory to probe the cluster properties of the stars in the Polaris complex. They relied on the fact that young, low mass stars emit relatively strong X-rays. Chandra has the sensitivity to spot any such small stars clustering in the Polaris group, and can distinguish between them and other kinds of X-ray emitting stars that might be seen in the same field-of-view.

In particular, Chandra can find stars that might be too faint (or too close to another star) to be seen in the optical, down to a mass potentially twenty times less than that of Polaris Aa. The presence or absence of small companion would help determine the history of the cluster, for example, if it ever had a close encounter with another star that could have perturbed the cluster.

In a new paper, the scientists report that they do not see such low-mass companions, but they conclude based on the X-ray properties that Polaris C and D are probably not physical members of the system, while Polaris B is a member, and is a single star.

Explore further: Los Alamos' sky-scanning software gives insight on events of astrophysical origin

Related Stories

There's More to the North Star Than Meets the Eye

Jan 09, 2006

We tend to think of the North Star, Polaris, as a steady, solitary point of light that guided sailors in ages past. But there is more to the North Star than meets the eye - two faint stellar companions. The ...

Cepheids and their 'cocoons'

Feb 28, 2006

Using ESO's Very Large Telescope Interferometer (VLTI) at Cerro Paranal, Chile, and the CHARA Interferometer at Mount Wilson, California, a team of French and North American astronomers has discovered envelopes ...

The Cosmic Distance Scale

Jan 29, 2010

(PhysOrg.com) -- In 1908, Harvard astronomer Henrietta Swan Leavitt discovered that a class of stars called Cepheids have brightnesses that vary regularly with periods that are directly related to their intrinsic ...

Moving Closer to the Grand Spiral

Aug 01, 2005

An international team of astronomers from Chile, Europe and North America is announcing the most accurate distance yet measured to a galaxy beyond our Milky Way's close neighbours. The distance was determined ...

Chandra Lifts the Veil on Milky Way 'Hotspot'

Jan 23, 2008

NASA's Chandra X-ray Observatory is helping to demystify Westerlund 2, a young star cluster with an estimated age of about one- or two-million years. Heavily obscured by dust and gas, Westerlund 2 has been ...

Recommended for you

Hubble observes one-of-a-kind star nicknamed 'Nasty'

10 hours ago

Astronomers using NASA's Hubble Space Telescope have uncovered surprising new clues about a hefty, rapidly aging star whose behavior has never been seen before in our Milky Way galaxy. In fact, the star is ...

Galaxy's snacking habits revealed

May 20, 2015

A team of Australian and Spanish astronomers have caught a greedy galaxy gobbling on its neighbours and leaving crumbs of evidence about its dietary past.

Supernova ignition surprises scientists

May 20, 2015

Scientists have captured the early death throes of supernovae for the first time and found that the universe's benchmark explosions are much more varied than expected.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.