"T Coronae Borealis"

Are we seeing a nova from the 13th century?

A white dwarf like this is currently making headlines.(Bild: Bartlomiej - stock.adobe.com)

A nova, literally a "new star", is set to light up in the Northern Crown constellation this year. The star has been around for a long time, but is rarely seen with the naked eye. Author and natural scientist Christian Mähr found a monk who had already observed the nova - in the 13th century. 

In any case, the star "T Coronae borealis" is still clearly visible through a telescope today. The system is around 2700 light years away and consists of two stars, one very large and one very small. They orbit each other at half the distance that the earth is from the sun.

The large one is a red giant, several times larger than our sun and "cooler". Not 6000 degrees, but half that or less. The small one represents the other extreme, a white dwarf. It has about the mass of the sun, but is only as big as the earth. It is therefore quite "dense" - there are ten tons in one cubic centimetre! How can two such different stars be so close together?