I've been looking at the OGLE Collection of Variable stars for the purposes of making a machine learning model to classify variable stars by their plots. In each variable star, I see the following image:
What does it mean? I know that the magnitude is the apparent magnitude of the star and err is the error, but what does HJD-2450000 mean? My guess would be a unit of time, but a quick google search didn't yield anything helpful.
HJD is the "Heliocentric Julian Date" HJD-2450000 is HJD minus 2450000
The Julian day is the number of days since the start of 4713 BC, with 0 assigned to the day of Monday, January 1st 4713 BC in the Julian Calendar. The Julian date is the Julian day + the fraction of the day since the previous UTC noon.
As I write the Julian date is (was) 2460509.809699
For events from outside the solar system, the observation of the event would depend on when the light from the event reaches the Earth. Since it takes about 16 minutes for light to cross the Earth's orbital diameter, this could create inconsistencies between the seasons: a variable star would appear to "speed up" as the Earth moved towards it. To adjust for these inconsistencies the convention is to describe the time of the event would be observed from the centre of the solar system, hence "Heliocentric Julian date.
But HJD is an inconveniently large number, so it is common to subtract a constant (eg 2450000). In your example 5265.85454 represents the HJD of 2455265.85454 or Wednesday 2010 CE, March 10 (Gregorian) 08:30:32.3(UTC) (with the light from the event reaching Earth with 8 minutes of that time)
