ASTRONOMICAL DISTANCE, MASS AND TIME SCALES

ASTRONOMICAL DISTANCE, MASS AND TIME 

SCALES 

In astronomy, we are interested in measuring various physical quantities, such as mass, distance, radius, brightness and luminosity of celestial objects. You have just learnt that the scales at which these quantities occur in astronomy are very different from the ones we encounter in our day-to-day lives.

Therefore, we first need to understand these scales and define the units of measurement for important astrophysical quantities. 

We begin with astronomical distances.

Astronomical Distances

You have studied in your school textbooks that the Sun is at a distance of about 1.5 × 10^11m from the Earth. The mean distance between the Sun and the Earth is called one astronomical unit. Distances in the solar system are measured in this unit. 

Another unit is the light year, used for measuring distances to stars and galaxies. 

The parsec is a third unit of length measurement in astronomy.

We now define them. 

Schematic diagram showing the definition of 1 parsec. Note that 1°=60' and 1'=60''. Thus, 1''= 1/3600 degree.

Dimensions of Astronomical Objects

The sizes of stars or stellar dimensions are usually measured in units of solar radius RΘ. For example, Sirius, the brightest star in the sky, has radius 2RΘ. The radius of the star Aldebaran. in Taurus is 40RΘ and that of Antares in Scorpius is 700 RΘ. 

Mass

Stellar masses are usually measured in units of solar mass MΘ. We know that MΘ = 2 × 10^30 kg. For example, the mass of our galaxy is ~ 10^11 MΘ. The mass of a globular cluster is of the order of 10^5 − 10^6 MΘ. S. Chandrasekhar showed  that the mass of a white dwarf star cannot exceed 1.4 MΘ. This is called the Chandrasekhar limit.

Time Scales

The present age of the Sun is about 5 billion years. It has been estimated that it would live for another 5 billion years in its present form. The age of our galaxy may be around 10 billion years. Various estimates of the age of the universe itself give a figure between 12 and 16 billion years. On the other hand, if the pressure inside a star is insufficient to support it against gravity, then it may collapse in a time, which may be measured in seconds, rather than in millions of years. 

The distances, sizes and masses of some astronomical objects

Next time when you look at the familiar stars in the night sky, you will have some idea of how far these are from us, and also how massive they are.