Stellar Distances

Cepheid Variables

Cepheid variables are very luminous stars that pulsate in a regular cycle, with rapid brightening followed by gradual dimming. They are named after the star delta Cephei, a naked eye star, which was the first of this type to be identified. Cepheids are relatively rare, but their unique properties enable scientists to measure the distance to stars in galaxies more than 10 Mpc away. Since it is very difficult to tell the difference between a light source that is far away and a dimmer source that is nearer to us, measuring the distance to other galaxies is one of the greatest challenges facing astronomers. Cepheid variables are a fantastic tool to help them.

Light Curves

The outer layers of a Cepheid variable star pulsate in a manner that is predictable. The outer layers of the star periodically expanding and contracting cause this pulsation.

Figure 3.6: Variation in star size and luminosity over time

Observations of Cepheids with well-known distances showed that a well-defined correlation exists between the average luminosity of a Cepheid star and its pulsation period. If a pulsating star, therefore, is detected in a distant galaxy, and it is identified as a Cepheid from its period and its spectral characteristics, its apparent brightness and its pulsation period can be used to determine its distance, which can also be defined as the distance to the cluster or galaxy in which it is found. Astronomers, therefore, have used the period-luminosity relationship to determine distances to galaxies.

Figure 3.7: Lightcurve for a classic Cepheid (SV Vul) - period of 44.96 days

Figure 3.8: Lightcurve for a classic Cepheid (SU Cas) - 1.949 days

You can find the light curves for these and other Cepheids on the Hipparcos web pages at:
http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=Epoch_Photometry

Period - Magnitude Relationship

To determine the average absolute magnitude for Cepheids, the following equation is used:

M = -2.78 log (P) - 1.35

Where M is the absolute magnitude of the star and P is the period measured in days.

Last Update: 14 May 2013

Related Articles

• Introduction (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1659)
• The parsec (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1660)
• Stellar Parallax (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1661)
• Distances using parallax (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1663)
• Apparent & Absolute Magnitude (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1868)
• Luminosity from Stellar Spectra (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1667)
• Cepheid Variables (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1671)
• Cepheids as 'Standard Candles' (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1673)

Related Links

• Hipparcos Light Curves (http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=Epoch_Photometry)
• The Hipparcos and Tycho Catalogues (http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=Overview)
• Access the catalogues (ESA) (http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=Research_tools)
• Access the catalogues (CDS) (http://cdsweb.u-strasbg.fr/hipparcos.html)

See also

• Hipparcos Mission (http://sci.esa.int/science-e/www/area/index.cfm?fareaid=20)
• Gaia Mission (http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26)

Documentation

• Hipparcos Star Data (http://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=35911)
• Teacher Notes 3 (http://sci.esa.int/jump.cfm?oid=37855)