What Is A Guest Star?
A “Guest Star” in astronomy is a star that just suddenly appears in the sky, in a location where there was not one before.
To be more accurate, the star WAS there before, but it was too faint for us to see. So it appears, not because it has been created, but because it suddenly becomes much brighter and we are able to observe it.
The period of massively increased brightness seems to last for 1-2 years, usually.
Why Are They Called “Guest Stars”?
These phenomenon are called guest stars because this is a literal translation of the Chinese Mandarin word that was used for them.
Why the Chinese? Well this is because astronomers in China over the last 2000 years, have been very good at observing and more importantly, recording in documents, when these sorts of happenings have occured.
There are fabulous observational archives in China which are only now being translated, so there may be a lot more historical knowledge to come over future years.
In Europe and elsewhere, guest star events have only been recorded in the last few hundred years.
What Causes A Star To Suddenly Get Brighter?
Guest stars are basically star explosions. There are two types:
a) Supernovae – when the star completely explodes and leaves nothing but dust and debris, and;
b) Novae – when an outer layer of the star, somehow “blows-off” with a great release of energy, but the remainder of the star survives.
It has been known for decades that the larger stars (perhaps those more than about eight times as massive as our Sun), can behave in this way.
Astronomers are now trying hard to classify stars into various types and so understand what circumstances give rise to nova and supernova events.
How Easy Are They To See?
They can be extremely easy to see, even with the unaided eye and sometimes even in daylight! However, for a supernova to be this bright, it has to be within our own galaxy and so, relatively close.
There has not been a guest star in our galaxy for over 400 years.
The last exploding star in our galaxy was the so-called “Kepler star” of 1604. This was in the constellation Ophiuchus and was as bright as the planet Venus appears.
Kepler did not discover this supernova, but he was an eminent professional astronomer at the time and he carried out accurate measurements of it.
Find a catalogue of known guest stars in our galaxy here Historic Catalogue Of Supernovae
Many, many star explosions have been observed in OTHER galaxies, especially in the last few years.
UK amateur astronomers deserve a mention here.
A few individuals have become particularly good at discovering supernova events in other galaxies, with over 200 found from UK since 1996. But these extra-galactic star explosions are of course much further away and require good telescopes to observe them.
Are Guest Stars Useful?
The short answer is, Yes.
As well as being dramatic, mind-boggling events that get people interested in astronomy, these supernova events are becoming increasingly useful to professional astronomers. They are making use of supernovae to estimate how far away distance galaxies are from the Earth, and so better understand the structure of the Universe.
