Source of the Observation of Moving Stars
From data that we gathered about 12 years ago (2005) using the International Space Telescope (IST) and data being gathered now (2017) by Gaia, some astronomers calculated the motion of various stars in the Dwarf Galaxy named Sculptor.
Gaia’s mission is to observe stars in the Milky Way (our galaxy.) Yet while doing so it records images with stars of the Dwarf Galaxy in the background.
Because we are limited to telescopes to look at other stars, we get an idea of their speed of motion going away (Redshift) or coming closer to us (Blueshift). The wave Doppler Effect was discovered by Christian Doppler in 1842. Christian described the phenomenon of a wave changing frequency when the source of that wave is in motion compared to the observer.
What is the Doppler Effect
As an example we often use the ambulance sirens that seemingly change their pitch between the time they approach you and the time they move away from you. Note, however, that Christian Doppler described the effect while describing colors changes in binary stars. It is Buys Ballot who first tested the theory with sound in 1845 and confirmed the Doppler effect. In 1848, Hippolyte Fizeau discovered the same phenomenon on electromagnetic waves without knowing of Doppler’s earlier discovery and Ballot’s tests with sounds.
Anyway, the ambulance sound changes for you, the observer, because the wave length is compressed as the ambulances approaches you. It can be explain from the fact that the next sound wave crest is emitted closer to the previous sound wave crest and thus reaches you sooner, increasing the frequency of the sound (i.e. higher pitch).
Now, when the ambulance drives away from you, the opposite happen and the wave length gets elongated. The next sound wave crest is emitted further away from the previous sound wave crest, decreasing the frequency of the sound (i.e. lower picth).
Of course, the people inside the ambulance do not hear that change in pitch since their position remains static compared to the location where the sound is emitted.
There is another interesting case with sound waves: it is possible to travel faster than sound (supersonic travel). In this case the source of the sound precedes the sound, although new waves still form right behind the source. An observer in front of such a source hears nothing until after the object passed him. An observer behind the source still hears the sound with a decreasing/lower frequency.
Also sound requires a medium to travel through. On Earth, it travels through the atmosphere. So the motion of the sound is also affected by the wind since movements of the transportation medium is going to affect the wave lengths.
So there are three things that change the pitch of sound on Earth, the movement (or lack thereof) of the observer, the movement of the source, and the movement of the medium transporting the waves.
Certain other waves, such as light and gravity¹, do not require a medium to travel through. In these specific cases, only the observer and source are taken in account to do our calculations.
Redshift and Blueshift
When applied to astronomy, the Doppler Effect is often mentioned as the Redshift or the Blueshift. As light frequency increases, it becomes redder and when it is compressed, it becomes bluer.
Although you have to be careful., There are actually two reasons why we observe a Redshift:
First we have stars receding just because of their location compared to us.
Second, the universe is believed to expand. This also exacerbate the Redshift. That is, from our observations, we have determined that many stars and galaxies are moving away by a constant amount all around us. This is explained by the concept that the Universe is still growing. The further away a galaxy is, the higher its Redshift is (the distance from us versus the Redshift are clearly proportional.)
Since we determined that cosmological constant, we can take it in account to determine whether a star is approaching or receding.
Additional Areas where the Doppler Effect is Used
The Doppler Effect is used in:
- Various Radar Technologies (to measure the velocity of the detected objects, hearing that siren again?)
- Medical Devices that use echocardiogram use the Doppler Effect to detect the direction and velocity of the blood flow. This technique uses Ultrasounds which would need to be at a perfect right angle from the blood flow to be 100% accurate. We’re generally close enough though and that’s better than having to open your veins. Note that some of these medical devices do not use the Doppler Effect, instead they measure the amount of time for the Ultrasound to do a round trip.
- Flow Measurements can be accomplished using systems such as the LASER Doppler Velocimeter (LDV) and Acoustic Doppler Velocimeter. These instruments are non intrusive making them quite attractive in many situations.
- Satellite Communication is affected especially for a fast moving Satellite. We have systems that compensate by sending signals that change frequency as required (i.e. the frequency has to change depending on the position of the Satellite when it is going to receive that signal.)
Moving Toward or Away
So the Doppler Effect allows us to determine whether a star is moving toward us (Blueshift) or away from us (Redshift).
This is a very interesting side effect of wave frequency. That specific movement is like carved in the wave before it reaches us and therefore the information is constantly available to us.
However, lateral movement cannot be detected without taking pictures of the same object over time…
So a group of scientists decided to make use of old data from 2005 and more recent data from 2017 to see various object moving laterally. This is the very first time that we had pictures of another galaxy with enough precision allowing us to calculate the expected position of stars and verifying that they indeed moved to that new location.
Unfortunately, that was not the case. The position definitely changed in the right direction, but not in the expected quantity. Further testing shows that our current theory of dark matter does not hold up very well at that location. (We think that dark matter surrounds galaxies, such as ours, forming gigantic halos.)
Note that was no easy task since the positions of the two telescopes were quite different. It was necessary to calculate the position of all the stars in an ever moving world.
In the end, the team of astronomers lead by Davide Massari found 15 stars that they were able to accurately track between the two observation periods.
Further calculations will be done to see whether the movements could be explained by the fact that the 15 stars used for the calculations are all within the same group of stars or not. From our current knowledge, there would be at least two groups in that galaxy. If all 15 stars are in the same group, then the dark matter theory is likely to hold as it is.
As the astronomers were working on the stars of the Sculptor dwarf galaxy, they also calculated the movement of the galaxy as a whole. Something to do to reach a speed at which each star moves compared to their galaxy center.
The fact is that we have been thinking that the galaxy would collide with the Milky Way but the new findings show that it will actually miss it, at least for quite a while longer.
The shape of the Sculptor galaxy was also thought to be the effect of previous passages of that galaxy near the Milky Way but the more precise trajectory calculated contradict that information. It looks like the Sculptor galaxy would not be disturbed but us that much. This means we’ll have to go back to the blackboard to find the real reason for the shape of that galaxy.
What do you think of such scientific discoveries? Let me know by commenting below!
¹ Note that most astrophysicists think that on September 14, 2015 we conclusively detected Gravitational Waves for the first time while observing two black holes merging. Henri Pointcaré first proposed gravitational waves in 1905. Albert Einstein predicted them in 1916 while working on his General Theory of Relativity. It is theorized that Gravitational Waves transport energy as gravitational radiation. It is thought to be similar to the electromagnetic radiation. Newton had not predicted the Gravitational Waves because he thought that gravity acted instantaneously on bodies (propagated at an infinite speed, to be precise.) Only Newton’s Classical Mechanics did not explain various discrepancies that are resolved by the General Theory of Relativity. In Einstein theory, Gravitational Waves move at the speed of light, which is the fastest anything can move in the Universe (although since the Universe is in expansion… but that will be a different discussion.)
Credits: The photo at the top of the Sculptor Dwarf Galaxy was taken by the European Southern Observatory (ESO, also called La Silla Observatory) found in Chile using its Wide Field Imager camera. This is a telescope with an aperture of 2.2 meters.