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: Black hole chokes on a swallowed star

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

Black hole chokes on a swallowed star

7 hours ago

A five-year analysis of an event captured by a tiny telescope at McDonald Observatory and followed up by telescopes on the ground and in space has led astronomers to believe they witnessed a giant black hole ...

Swarm of microprobes to head for Jupiter

13 hours ago

A swarm of tiny probes each with a different sensor could be fired into the clouds of Jupiter and grab data as they fall before burning up in the gas giant planet's atmosphere. The probes would last an estimated ...

A recoiling, supermassive black hole

18 hours ago

When galaxies collide, the central supermassive black holes that reside at their cores will end up orbiting one another in a binary pair, at least according to current simulations. Einstein's general theory ...

Chandra celebrates the International Year of Light

Jan 23, 2015

The year of 2015 has been declared the International Year of Light (IYL) by the United Nations. Organizations, institutions, and individuals involved in the science and applications of light will be joining ...

Why is Andromeda coming toward us?

Jan 23, 2015

I don't want to alarm you, but there's a massive galaxy heading our way and will collide with us in a few billion years. But aren't most galaxies speeding away? Why is Andromeda on a collision course with ...

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.