Is this measured by parallax and also do they account for how much it has moved since the light was emitted? If it measures 2 million light years away it’s moved a lot closer in those 2 million years right?
Anonymous
It could have moved further away or stayed in the same position. Your reasoning is uncomprehensive.
?
Of course it wasn't measured by parallax. That doesn't work for distances of more than about 50 light years. And as that distance was measured less than a century ago, it hasn't moved any significant distance since. Bear in mind it'll take another 4 billion years or so before it collides with the Milky Way.
Ronald 7
I wouldn't argue with that
Those guys know their Mathematics
Raymond
The distance is actually closer to 2.5 million light-years, but the idea remains the same: there are certain types of stars where you can know the proper luminosity (the amount of light emitted) from certain properties. One example is the variability period of a Cepheid star (a type of variable star). By looking at a few Cepheids in the other galaxy and observing their period, you have their luminosity. Compare that with the apparent brightness of the star and you can find the distance.
In astronomy, the distance is given based on the time light takes to go from the object to us (look-back distance). Therefore it is true, in this particular case, that the Andromeda galaxy "now" is slightly closer to us than what we can see. We know the radial speed (how fast it is approaching us) so that we can calculate the real distance where it would be IF we could see it where it is now (i.e., if light was instantaneous). This distance is called co-moving distance.
The radial relative speed of that galaxy is approximately 300 km/s. This sounds fast but space is big. Given the uncertainty in the total distance (2.5 million light-years), if we subtract the 2,500 light-years it would have approached us over that time of 2.5 million years, we'd still call it 2.5 million light-years in co-moving distance.
2,540,000 light-years (look-back distance)
- 2,500 light-years (approach over 2.5 million years)
2,537,500 light-years (co-moving distance)
billrussell42
no, parallax does not work at those distances, not even close.
reference has more detail, At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy.
yes, the measurements show the distance 2 million years ago.
The Andromeda Galaxy is approaching the Milky Way at about 110 km/s, it is expected to collide directly with the Milky Way in about 4 billion years.
given the above, 4 billion year to collide, 2 million LY away, the distance number is not that far from the actual number.
Parallax:
To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit. It is only able to measure parallax angles for stars up to about 1,600 light-years away,