What are Cepheids?

Cepheids, also called Cepheid Variables, are stars which brigthen and dim periodically.

In fact, Cepheids are very special variable stars because their period (the time they take to brighten, dim and brighten again) is

1 . Regular (that is, does not change with time) and

2 . A uniform function of their brightness. That is, there is relation between the period and brightness such that once the period is known, the brightness can be inferred. Brighter the Cepheid, the longer its period.

Must read: Major Stages of the Life Cycle of Stars

When was the first Cepheid variable discovered?

The first Cepheid variable to be discovered was Eta Aquilae, which was observed on September 10th, 1784, by English astronomer Edward Pigott.

What are the uses of Cepheids in astronomy?

The variable behavior of Cepheids allows them to be used as cosmic yardsticks out to distances of a few tens of millions of light-years.

The Cepheids are reasonably abundant and very bright. Astronomers can identify them not only in our Galaxy, but in other nearby galaxies as well. If one requires the distance to a given galaxy one first locates the Cepheid variables in this galaxy. From these observations one determines the period of each of these stars. Leavitt’s data states that a given period has a unique brightness associated to it.

So from the period and Leavitt’s plot we get the brightness at the distance of one light-year. We can also measure the brightness on Earth. The brightness at the distance of one light-year will be larger than the observed brightness due to the fact that brightness drops like the square of the distance. From these numbers one can extract the distance to the stars. This method works up to 13 million light-years when Earth-bound telescopes are used; for larger distances these stars become too dim to be observed.

Recently, space-based telescopes such as the Hubble Telescope, have used these stars to much farther distances. Looking at a galaxy in the Virgo cluster called M100, astronomers used the Cepheid variables observed there to determine its distance – 56 million light-years.

What are the different classes of Cepheids?

Cepheid variables are divided into two subclasses – Classical Cepheids and Type II Cepheids – based on differences in their masses, ages, and evolutionary histories.

Classical Cepheids

Classical Cepheids are Population I (metal-rich) variable stars that are 4-20 times more massive than the Sun and up to 100,000 times more luminous. They undergo pulsations with very regular periods on the order of days to months.

These Cepheids are typically yellow bright giants and supergiants (spectral class F6 – K2) and they experience radius changes in the millions of kilometers during a pulsation cycle.

Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble Constant can be established.

Type II Cepheids

Type II Cepheids are Population II (metal-poor) variable stars which pulsate with periods of typically between 1 and 50 days. Type II Cepheids are also older stars (~10 billion years) that have around half the mass of our Sun.

Type II Cepheids are also subdivided based on their period into the BL Her, W Virginis, and RV Tauri subclasses (named after specific examples) – which have periods of 1-4 days, 10-20 days, and more than 20 days, respectively.

Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, and neighboring galaxies.

Anomalous Cepheids

There are also those that do not fit into either category, which are known as Anomalous Cepheids. These variables have periods of less than 2 days (similar to RR Lyrae) but have higher luminosities. They also have higher masses than Type II Cepheids, and have unknown ages.

Double-mode Cepheids

A small proportion of Cepheid variables have also been observed which pulsate in two modes at the same time, hence the name Double-mode Cepheids. A very small number pulsate in three modes, or an unusual combination of modes.

PRACTICE QUESTIONS

QUES . Consider the following pairs : UPSC PRELIMS 2023

Objects in space : Description

1 . Cepheids : Giant clouds of dust and gas in space

2 . Nebulae : Stars which brighten and dim periodically

3 . Pulsars : Neutron stars that are formed when massive stars run out of fuel and collapse

How many of the above pairs are correctly matched?

(a) Only one

(b) Only two

(c) All three

(d) None

Show Answer