Saturday, 29 August 2015

Astrognome Scrapbook Novae Cygni 1975

Nova Cygni 1975

The constellation of Cygnus the Swan dominates the sky in the northern hemisphere during summer months, Cygnus is sometimes known as the Northern Cross. Deneb its brightest star is part of the summer triangle of bright stars overhead during the summer months. However on the night of August 29th 1975 the shape of Cygnus changed for a short period of time when a Nova appeared.

The term Nova comes from the Latin for New, a few hundred years ago when astronomers saw what we call Nova they thought they were new stars being created. Hence the name Nova.

A nova is a binary system where two stars orbit each other. Typically one will be a white dwarf, and the other a red giant, the white dwarf has a strong gravitational field and pulls gas from the larger though less massive red giant. When some of this less dense and cooler gas falls onto the hot surface of the white dwarf it is thrown off into space, the star will become brighter for a period of time before returning its normally brightness. This process can happen more than once.

Many independent discoveries were made of the Nova at magnitude 3, the following day it rose to magnitude 2, which is as bright as the North Star. This makes it very easy to see. It quickly faded, over the next three days it faded to magnitude 5 towards the edge of naked eye visibility. During the next few weeks it faded to magnitude 9 making it a difficult object to find in binoculars.

At the time of its discovery it was simply referred to as Nova Cygni 1975, today it has the designation nova V1500 Cyg, which means it was the 1500 variable star to have been discovered in the constellation of Cygnus.

It was the brightest novae since 1942 and there have been no novae as bright as this seen in our galaxy since Nova Cgyni 1975. Today a powerful telescope is needed to find.

Where have all the bright Novae gone? During the 20th century  6 novae were seen between 1900-1950 which were at least as bright as the North Star, from 1950-1999 there was only 1. In the 21st century so far there have been none.

I wonder when the next bright Nova will be seen, the next time you go out to observe take a few minutes to look at the bright stars that form the constellations to make sure there are no ‘new’ stars there before you go onto hunting those very faint objectives that attract all the attention today.

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