- Astronomers measure distances to galaxies using Cepheid stars
- Their brightness is used as a cosmic ‘standard candle’
- Method brings galaxy NGC 247 one million light-years closer
HOW FAR AWAY is this galaxy? According to astronomers, it is a lot closer than previously thought—in fact, about a million light-years closer.
Distances to nearby galaxies are measured using the properties of a particular kind of star known as a ‘Cepheid variable’.
Cepheids brighten and fade with a regular pattern. Astronomers can plot a Cepheid’s brighten/fade period and then put that into a special formula, which gives them the star’s intrinsic luminosity…that is, how bright it would be if we were up close to it.
By comparing that intrinsic brightness with the star’s actual measured brightness, astronomers can gauge how far away it is.
It’s a bit like being able to judge how far away a car is at night, based on how bright its headlights seem to be.
Astronomers refer to Cepheids as ‘standard candles’, and have used this method for many years to estimate the distances to nearby galaxies. It doesn’t work for very distant galaxies—as we look further out into space, a point is reached where individual stars cannot be made out.
But difficulties remain with the Cepheid method. Recent work has suggested that the Cepheids’ ‘period-luminosity’ relation is not as clear-cut as previously thought.
In addition, a Cepheid’s brightness can be affected by interstellar dust absorbing some of the star’s light, making it seem fainter than it really is.
Dimmed by dust
NGC 247 is a spiral galaxy in the Sculptor Group of galaxies, which is the nearest collection of galaxies to the Milky Way’s own galaxy cluster, the Local Group.
But how far away is it?
One of the good things about NGC 247 is that it is close enough that individual stars can be made out in high-resolution images, making it a prime candidate for Cepheid measurements.
But the galaxy is tilted to our line of sight—we’re seeing it about halfway between face-on and edge-on. This means that its starlight, including that from the Cepheids, has to pass through a lot of the dust inside the galaxy.
An international team of astronomers known as the Araucaria Project is seeking to refine NGC 247’s Cepheid distance measurements by taking the dust into account.
Their initial findings suggest NGC 247 is over 1 million light-years nearer to us than earlier thought. The official distance now stands at a little over 11 million light-years.
The image was produced by combining a number of exposures taken (through different filters) with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the European Southern Observatory’s (ESO) La Silla Observatory in Chile.
One of the filters brings out clouds of hydrogen gas (coloured pink) along the spiral arms, indicating areas of active star formation.
Written by Jonathan Nally, SpaceInfo.com.au. Image courtesy ESO.
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