By observing the sky every night and seeing the stars, we can appreciate that they remain static in the sky, but is it really like that or not?
This seems so to us given that there is a great distance between all of them and between us and the sky, but when observing carefully and comparing for long periods of time if the stars are moving or are fixed , we see that their position has varied throughout the history.
The proper motion of the stars
Before the main question that has brought you to this article, that is, to know with certainty if the stars are moving or are fixed , we clarify that they move, but generally they do so in such a way that it is difficult to perceive it.
In 1718, the astronomer and physicist Edmund Halley was the one who first verified the displacement of the stars . He did this by comparing the position of three of the brightest stars , named Procyon, Arthur, and Sirius . He found that its position had varied in relation to the neighboring less bright stars, specifically, 0.5º for Sirius and 1º for Arthur.
The apparent displacement of stars in the sky is called proper motion and is measured in arc seconds per year (“/ year). Comparing two snapshots of the same area of the sky, measured with a period of difference of 50 years or more, it is possible to check the displacements of the stars perpendicular to our view. This is the proper motion of the stars projected in the sky.
In general, these movements are of very small dimensions , since the vast majority of stars have their own movements of the order of 0.0001 “/ year, with the exception of a few stars that can reach 1” / year. Barnard’s star represents a very striking case, since it reaches 10.25 ”/ year, the equivalent of 1º every 350 years.
The proper motion of a star can be divided into several components. The tangential velocity of the stars is measured perpendicular to the observer’s line of sight, knowing the distance a star is located and the radial velocity measures whether it approaches or moves away from the observer. The direction of displacement of a star can be deduced geometrically from the ratio between its radial and tangential velocities
The radial velocity of the stars’ motion is the component that develops along the observer’s line of sight. This speed is measured with the spectral lines of the stars, which shift to blue or red as the observer moves away from or closer to the light source ( Doppler effect ). This consists of measuring the spectrum of the star in superposition to a terrestrial source. Generally, it is measured in km / s. This can be of approach (negative measures) or distance (positive measures), according to its movement towards blue or towards red. After measuring this speed in a large number of stars, most of them have speeds between 10 and 40 km / s, with some exceptions that reach 100 km / s.
Radial velocity can also tell us about the physical characteristics of some stars. Thus, in double stars their radial speeds have periodic variations that characterize their orbital movements. In other stars, called pulsating, these variations show the expansion and contraction of their surface.
Space velocity of the motion of the stars
The third component of a star’s motion is space velocity . Actually, this component can be calculated based on its radial (Vr) and tangential (Vt) velocity:
- Ve 2 = Vr 2 +Vt 2
Where Ve measures space velocity in relation to the observer. If we subtract the speed of the observer, we can obtain the absolute speed. The brightest star in the sky, called Sirius, has a radial speed of -8 km / s.
By analyzing the spectral lines of the stars, it is also possible to know their speed of rotation. In this spectrum, the thin lines would indicate a low speed of rotation, while the wider lines would indicate a high speed of rotation. Not only this, but the width of the lines also determines the position of the axis of rotation with respect to the observer’s vision.
Thus, if this axis is perpendicular to the vision, we will be able to obtain the real value of the rotational speed , while if it coincides with the visual of the observer, it would not be possible to determine its real value.