Cepheid variable stars are most commonly known as a standard candle for measuring galactic distances. That’s because they vary in brightness at a rate proportional to their average brightness.  But they can also tell us something about how young stars are distributed within our galaxy, and a recent study raises an interesting mystery. 

There are basically two types of Cepheid variable stars. Classical cepheids are large bright stars, typically with a mass 5 – 20 times that of our Sun. Since larger stars have shorter lifetimes, a classical Cepheid is typically no more than 100 million years old. Type II Cepheids are small, old stars, with masses much less than our Sun. They are typically around 10 billion years old. Distinguishing between these two types of Cepheid variables is straight forward, because they have very different metallicities (traces of elements other than hydrogen and helium). So we can distinguish them by looking at their spectra and the way they brighten and dim (their light curve). Because classical Cepheids are brighter, they are typically used to determine the distances to galaxies, and helped establish the Hubble law for cosmic expansion. The dimmer type II Cepheids are typically used for distances within our galaxy, such as determining the distance to the center of our galaxy.

Since the ages of these different types of Cepheids are very different we can use them as a gauge for the age of surrounding stars. For example, if a globular cluster contains type II Cepheids, we know it is billions of years old. If a star cluster contains a classical cepheid, we know that stars formed there relatively recently. A new paper in MNRAS uses this fact to look at the distribution of young stars in our galaxy, and found a rather puzzling void.

Mapping young stars in our galaxy can be a challenge, particularly in the direction of the center of our galaxy, where high amounts of gas and dust obscure most of the visible light from distant stars. Fortunately infrared light isn’t absorbed as strongly, so an infrared survey of Cepheids gives us a good view of the central region of the Milky Way. This new study found some classical Cepheids clustered very close to the center of our galaxy, but found a region about 8,000 light years in radius where there aren’t any classical Cepheids. This would seem to indicate that this region hasn’t produced stars in at least 100 million years. This is in agreement with infrared and radio surveys of the central region of our galaxy, which also find a lack of star producing regions in that area.

We don’t know why stars don’t form in this region. There is certainly plenty of matter in the region, and older stars are clearly present there. For some reason the conditions for young stars are lacking there, producing a cosmic “get of my lawn” effect.

Paper: Noriyuki Matsunaga, et al. A lack of classical Cepheids in the inner part of the Galactic disc. MNRAS 462 (1): 414-420. (2016) doi: 10.1093/mnras/stw1548

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In the late 1800s Henrietta Leavitt was hired by Edward Pickering of the Harvard College Observatory. “Hired” in this case being a loose term, since Leavitt was not initially paid for her work. She was assigned the task of cataloging the brightness of variable stars from photographic plates. This is a tedious process, which is why it was done by women (known as Pickering’s Harem).

Period vs luminosity for variable stars. Credit: ATNF

As Leavitt cataloged thousands of variable stars in the Magellanic clouds, she noticed a particular trend in a type of variable known as a Cepheid variable, specifically that the rate at which the star varied (its period) correlated with the apparent brightness of the star. By assuming the stars in a particular Magellanic cloud were all essentially the same distance, she was able to demonstrate a linear relationship between absolute brightness (luminosity) and period, seen in the figure here. She published her results in 1912.

A year later Enjar Hertzsprung measured the distance of several Cepheid variables in our a galaxy, and confirmed Leavitt’s period-luminosity relation. This meant Cepheids could be used as “standard candles”. By observing their variable period, one could determine their absolute brightness. Comparing this to their apparent brightness, one can determine their distance. This allowed Edwin Hubble to use observations of Cepheids in the Andromeda galaxy to confirm that it was not simply a nebula, but a galaxy millions of light years away. This confirmed that the Milky Way was also a galaxy, and revolutionized our view of the universe.

Today we know of two main types of Cepheids, defined by their metallicity (which we can determine from their spectra). There is also another type of variable known as RR Lyrae, which are smaller and have shorter periods. Their use as standard candles is critical to our determination of the scale of the universe.

All through the standard behavior of certain variable stars.

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While we’re quite familiar with our side of the Milky Way galaxy, the far side of our galaxy is still a bit of a mystery.  The reason for this is that the center of the Milky Way is filled with gas, dust and stars, so it is very difficult to see the other side of our galaxy.  The central region is so cluttered with material that it sometimes referred to as the Zone of Avoidance, since we have to exclude that region from observations beyond our galaxy.  

Map of neutral hydrogen in our galaxy.

There are some things we can observe about the far side.  While the central region blocks most of the visible light from the far side, it doesn’t block as much of the radio waves, infrared and x-ray wavelengths.  So we have been able to make some broad observations.  We know, for example, that our galaxy really is a barred spiral galaxy because we can map the distribution of hydrogen throughout most of the far side.  We can also see some of the spiral arms that exist on the far side.

Another way to observe to far side is to focus on the outer region where material flares out from the central plane of the Milky Way.  It is sometimes referred to as the flared edge of the galaxy. We’ve mapped gas and dust in that region, but now a new paper in Nature has announced the observation of Cepheid variable stars in this region.

Cepheid variables are a particular type of star that oscillate in brightness proportional to its absolute magnitude. They are extremely useful because you can use their observed variation to determine their actual distance.  So by observing these Cepheid variable stars, the authors could show conclusively that they are, in fact, within the flared region of the far side.  This is important because observing their location and motion can help us better determine the overall motion of stars in our galaxy.

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A while ago I wrote about how you can use depth perception (in astronomy we call it parallax) to measure the distance of nearby stars. While this works well, it only works if the stars are closer than about 1500 light years. So how do we measure more distant objects, such as nearby galaxies which are millions of light years away? For that we use an interesting type of star known as a Cepheid variable.

Cepheid variables are stars that vary in brightness over a period of days. The first such star to be observed was Delta Cephei in 1784, hence the name. For nearby Cepheids, we can determine their distance via parallax. We can also determine their apparent magnitude, and given their distance we can determine their absolute magnitude. I’ve written about the relation between apparent brightness and distance earlier.

It turns out there is a linear relationship between the average brightness of a Cepheid variable star (its luminosity) and the period at which its brightness varies. In the figure below I’ve plotted the luminosity of some Cepheid stars vs their period. You can see there is a nice linear relation between them.

Brightness vs period for Cepheid stars.

What this means is that if you determine the period of a Cepheid variable you can calculate its absolute magnitude. By measuring its apparent magnitude you can calculate its distance. From the Hubble telescope we have observations of Cepheid variables in nearby galaxies. From this we can measure galactic distances up to about 100 million light years.

So by using parallax we can determine the distance of nearby stars. We can also prove the Cepheid variable relationship. From the Cepheid relationship we can determine distance to nearby galaxies. For more distant galaxies we have to use a different trick involving supernovas, but that is a story for another time.

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