Stars move?
Yes, they move very fast but their motion appears very small because they are so far away.
The stars in our neighborhood are drifting along with us in an arm of the Milky Way as we all orbit the center of the Galaxy. Many stars move along with their neighbors and we assume they have a common origin from the same nebulae. However, some stars are moving faster than others and there is some relative motion even among the stars in our neighborhood.
Notice I said "relative
motion". In space it's difficult to tell whether you are moving or the object you are observing is moving. There is no fixed reference point. Here on Earth we have the ground as our foundation (literally) on which to measure motion but there is no such "rock solid" reference point in space, especially when talking about stars. A star may appear to be moving towards us when, in fact, it might be that the star is stationary and we are moving towards it. Even more complicated would be the situation where a star is moving away from us but might appear to be moving towards us if we are moving towards it faster than it is retreating! I hope you can see my point. (Space motion will make you sick! ) In astronomy we often say something is moving towards us or away from us but that assumes we are stationary and that other object is moving. In fact, everything in space is moving and that makes it very difficult to get a handle on motion if we try to measure it in absolute "rock solid" terms because we have no stationary reference point. Instead we talk about "relative motion" - the net (overall) effect of our motion and the motion of the object we are talking about.
When we and other astronomers speak of motion we usually are talking about "relative motion" but, to keep the conversation shorter, we usually simply call it "motion" - as if we were stationary and the other object was doing all the moving.
Relative motion is a very important concept in astronomy. We will come back to it next month when I teach you about the origin and fate of the universe. Also, relative motion is a corner stone of some advanced areas of physics, which we will not discuss but which you might want to explore in your own time.
The actual motion of a star is a combination of its transverse and radial motions. This requires some trigonometry in order to do the actual calculations but you don't need to go that deep into the math in order to appreciate star motion.
First let's discuss transverse motion.
Transverse motion (sometimes called "proper motion") is the motion we see as the star moves across the
sky. It is the motion PERPENDICULAR to our view. We can map the
position of a star against a field of stars and measure the change
it its position over the years.
Astrometry is a special branch of astronomy that measures the transverse motion of stars. Photographic plates, taken at different times (sometimes decades apart), are closely examined (nowadays by computers) to detect very small differences in the position of a star relative to its background stars. | ![]() |
In September 1916 the American astronomer Edward Emerson Barnard published articles in The Astronomical Journal and Nature exhibiting data which showed that a particular (small and otherwise insignificant) star has a tremendous transverse motion. He calculated that this star, which has come to be called "Barnard's Star", has a transverse motion of 10.3 seconds of arc per year. At that rate Barnard's Star will move across the sky a "distance" equal to that of the Full Moon in about two centuries.
Barnard's Star has the fastest transverse (proper) motion of any known star. Either it's moving very fast perpendicular to our field of view or it's very close. Or both. Using parallax measurements (which you learned about in the last lesson) it was found that Barnard's Star is only 5.8 light-years away. That makes Barnard's star our second closest neighbor. (The multiple star system of Alpha Centauri is our closest neighbor and I will tell you more about it next month.)
Barnard's Star is a red dwarf in OPHIUCHUS but has a relative magnitude
below 9 so it's impossible to see without a good telescope. That's why I haven't bothered trying to show you where it is. You already know how to find OPHIUCHUS. Suffice it to say that Barnard's Star is south of Rasalhague among that group of three stars that form a nice triangle on the eastern line of OPHIUCHUS. (But don't knock yourself out trying to find it!)
As you may have noticed I prefer to use the term "transverse motion" instead of "proper motion". Transverse motion is the correct term in math and mechanics for this kind of movement but many astronomers like to call it proper motion. I don't like to use that term because it can confuse students when used in combinations with relative motion and apparent motion. I like to use the expression "proper" to mean "true" or "obvious". (Besides, what's "improper motion"!? )
So Barnard's Star moves slowly across the star field in a straight line?
Yes. Ah, no. Well, maybe!
Barnard's Star does move across the star field slowly. If that was the whole story then the star should certainly travel in a straight line. Isaac Newton explained that an object in motion will remain in motion unless acted upon by an outside force. He also went so far as explain that an object should move in a straight line unless acted upon by an outside force. It's arguable whether Barnard's Star moves in a straight line. This brings us to an important area of astrometry because it leads us to the first attempts to detect planets around other stars. Barnard's Star might have a planet or two around it! This is controversial but it also makes for a good story and it will teach you how transverse motion, measured by astrometry, can be used to detect planets around stars.
If you think back to your lessons about the Moon you will recall the concept of a barycenter. If you were far off in space and observing the Earth with very sensitive measuring instruments, you could detect the way the gravitational attraction of the Earth and Moon causes them to rotate around their common center of gravity - the barycenter. This effect would be best seen from a position high above the orbital plane of the Moon - that is, if the pair are viewed such that the orbit is perpendicular to your view, you would see maximum wobble of the Earth-Moon pair. The pair would sway from side to side as they orbited the Sun. Of course, the major motion would be the orbital motion around the Sun but superimposed upon that otherwise perfect (elliptical) orbit would be a snaking of the pair as they orbited around their barycenter.
Now imagine applying this concept in astrometry to detect the wobble of a star. A star with a very large planet, especially one in a close orbit, will wobble as the star and planet waltz around their barycenter. The star, of course, will be much brighter than the planet because the planet only reflects the star's light. Therefore, it is only the motion of the star that is recorded and studied in this technique.
As the planet (in blue) moves in its orbit (in green) the star (in yellow) will move around the system's barycenter. If you plot the motion of the star against its background starfield you would see that it does not follow a straight line as it should (the line in gray) but instead it gently swings back and forth in a sinusoidal wave (in yellow). Most people call this a "wobble" but professional astronomers (at least when speaking among their peers) use the more scientific expression of a "perturbation" to describe any abnormalities in a star's motion.
Peter van de Kamp, at the Sproul Observatory of Swarthmore College, spent most of his life studying over 2000 plates (astronomical photos) of Bernard's Star taken by him and his students between 1938 and 1962. He claimed to have detected a perturbation in Bernard's Star. The sinusoidal wave had a period (repeated itself) of 24 years, so he conclude that Bernard's Star's invisible companion has an orbital period of 24 years. Careful measurements of the amount of displacement (how far away the wave deviated from the straight line), along with calculations based upon Newton's laws of gravity and motion, caused van de Kamp to conclude that the mass causing this sway in the path of Bernard's Star has a mass 1.6 times that of Jupiter.
Throughout the 1960s and 1970s van de Kamp refined and recalculated his work. He concluded that there was a second planet, with a mass of 0.8 that of Jupiter and an orbital period of 12 years, also orbiting Bernard's Star. In his most recent report of 1985 van de Kamp concluded that the planets had orbital periods of 12 and 20 years with masses of 0.7 and 0.5 the mass of Jupiter. He claimed that this new data were based on more observations (two more decades worth) and a rethink about how he was using the reference stars to do his calculations.
Of course, other astronomers were keen to confirm van de Kamp's work but they could not! In 1973 Gatewood and Eichhorn, reported they did not detect any wobble in Bernard's Star. Meanwhile, John L. Hershey, working at the Sproul Observatory, analyzed the same plates that van de Kamp had used. He also studied 12 other stars on the plates. Hershey detected the wobble in Bernard's Star but also found wobbles in the other 12 stars he had chosen at random! Either all 13 stars had companions (unlikely) or something was wrong with the Sproul Observatory telescope. Other astronomers have made many observations of Bernard's Star. Some claim to have detected a wobble but even these observations are hard to confirm.
Does Bernard's Star have a wobble? Does Bernard's Star have a dark companion? I don't know. No one knows. Van de Kamp went to his grave in 1995 still defending his beliefs. He explained that none of his critics had as much data as he had collected, that he had taken account of errors in the observational data and that, if others only studied his raw data in as much depth as he had, that they too would believe there are two planets orbiting Bernard's Star.
I hope you will appreciate that astrometry is a difficult subject with inherent problems but the concept is a good one. All you need is lots of high quality data collected over long periods of time. On the other hand, it takes a big planet to tug a star noticeably around the barycenter. For example, if you were viewing the Sun from high above the plane of the Solar System at a distance of 30 light-years away, the Sun would wobble only 0.00000014 of a degree around its barycenter (caused mostly by Jupiter's tug). That works out to the size of a coin seen from a distance of 10,000 kilometers!
Sadly, no one has collected enough of such data on any star to conclude, from astrometry, that a planet orbits a star outside of our own Solar System.
However, other techniques (other than the wobble of astrometry) have detected planets around other stars and I will tell you about one of those other methods shortly. But first, a recap about transverse motion and what it means to you as an amateur astronomer.
You will understand that the motion of the Earth, its spin and orbit around the Sun, is responsible for the constantly changing positions of the stars, short-term (due to the Earth's rotation) and long-term (due to the Earth's orbit). You have learned to use Polaris as your point of orientation and to identify stars by the patterns and positions they have relative to each other. That is - you use a concept of a "star map" and you orient that map with Polaris. You learned this month that Polaris will not always be our North Star. Perhaps it has now occurred to you that transverse motion causes the star maps to change over time!
Don't let this worry you. Like precession (which changes our North Star) transverse motion affects the star maps only over very long periods of time. Certainly in your lifetime, the constellations will always be recognizable. However, every astronomy student should be aware that the star map will change. Professional astronomers, who use very accurate measurements of nearby stars, must keep transverse motion in mind when doing their research.
So, the constellations don't really change much.
Right! In your life the pattern of the stars will not change (greatly). Their transverse motion, measured in arc seconds per year, is too small to cause much of a change in our lifetimes.
Let's move on to the other motion - radial motion. Remember, space is three dimensional so the transverse motion of stars across the sky is only an indication of their motion in two dimensions - left or right (the X axis) and up or down (the Y axis).
The other form of star motion, radial motion,
provides information about a star's motion in the third dimension - towards or away (the Z axis).
This isn't really a different type of motion. It just looks different because of our perspective.
Radial motion is motion towards or away from the Earth.
That doesn't change the pattern of stars in the sky, but it is
an important feature of a star's motion and it is measured in
kilometers per second.
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Measuring radial motion is not as straight forward as measuring transverse motion. A star looks like a tiny point of light so you really can't notice it getting bigger or smaller as it moves toward or away from you. Stars are too far away for that kind of measurement. Instead we use a "trick" from spectroscopy!
You will recall (from your lesson on spectroscopy) that a star produces a complete spectrum from its photosphere but above the photosphere lies an "atmosphere" of low
pressure gasses called a chromosphere.
As the continuous
spectrum passes through the chromosphere photons of specific wavelength (energy) are absorbed by specific atoms and ions.
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All stars have their equivalent of Fraunhofer lines and we can use these absorption bands to detect radial motion of a star. Stars with sufficient radial motion have shifted absorption lines and the amount the lines are shifted indicates the radial speed. This is due to an important phenomenon in physics called the Doppler effect.
Perhaps you have noticed that a sound fast approaching you (say, a siren) changes its pitch as it passes by. This is the Doppler effect in relation to sound waves, but it can occur with any kind of wave including light waves.
If a green light (5000 Angstroms) is moving toward you (or you
are moving toward it) the length of each wave will appear shorter
because the motion causes the waves to arrive more quickly than
normal. This apparent shortening of the wavelength shifts the
color to that of a shorter wavelength. In this example the light appears blue. (Blue has a wavelength of approximately 4000 Angstroms.)
We say the light has been "blue shifted" due to the Doppler effect. Note that a blue shift does not necessarily mean the light is blue, only that its wavelengths have been shortened due to the relative motion. A blue shift can occur to any wavelength of light emitted by an object that is moving towards you (or you towards it). We like to call it a blue shift because we like to use the visible spectrum as a reference point and to remind us that a blue shift will make a green light bluer (as illustrated here). | ![]() |
What if the source of light is moving away from us?
If that green light is moving away from you (or you from it) each wave is stretched out by the motion so it arrives with its wavelength drawn out into a longer wavelength. You see the green light as redder than it truly is. We say it has been "red shifted" due to the Doppler effect. Again, a red shift does not necessarily mean that the light has become red. It only means that the wavelength of the light has been pulled longer, by the motion, so each crest of the wave arrives slightly behind where it should be. We use the visible spectrum to remind us that a green light undergoing a red shift will appear more red than it really is. | ![]() |
The Doppler effect causes a shift in the entire spectrum of a star including its absorption lines. The absorption spectrum of a star is full of lines and they are in very exact positions but the Doppler effect causes them all to be shifted.
For example, sodium's two yellowish lines (at 5896 and 5890
Angstroms) will be shifted to longer wavelengths if the star
is moving away from us. If the star is moving very quickly, sodium's
two absorption bands could be shifted all the way into the orange (if moving away from us, as shown here).
All the absorption lines of the star's elements are shifted in the same direction and to the same extent. | ![]() |
The direction in which the absorption lines are shifted depends upon the direction of
radial motion. If the object is moving towards us, all the absorption lines will be squeezed to smaller wavelengths and the light will be blue shifted. If the star is moving away from us, all its absorption lines will be stretched to greater wavelengths and the whole absorption spectrum will be red shifted.
The amount of shifting depends upon the speed. The faster the speed the greater the shift. Therefore, by carefully observing the amount of shift in a star's
absorption spectrum, astronomers can calculate not only whether
the star is moving toward or away from us but also at what speed.
Measurement's of the absorption lines from Barnard's Star indicate that it is blue shifted so it is moving toward us. Careful measurements of the amount of the shift indicate that it is approaching us at a speed slight faster than 100 kilometers per second.
You mean it's going to hit us?!
No. The radial motion (100 kilometers per second) is only part of the motion. The transverse motion of Barnard's Star must be taken into account too. Calculations of both radial and transverse motions show that Barnard's Star's closest approach will be in the year 11,800 (AD) when it will pass within 3.75 light-years of us (closer than Alpha Centauri).
It's worth mentioning that a star tugged about by an invisible companion can also display a small amount of shifting in its absorption spectrum due to the Doppler effect. When the companion moves behind the star ("behind" it from our perspective) the companion's gravity pulls the star back slightly and that causes a very (very) slight red shift in the star's spectrum. Later, when the companion has circled half way around in its orbit, the companion's gravity pulls the star slightly in our direction and we measure a very (very) slight blue shift in the star's spectrum.
If you think about the geometry here, you will understand that the best shifting occurs when the companion's orbit is along a plane to our perspective. That is, maximal shifting occurs if the orientation of movement is 100% radial.
In the image below I have shown the angle as slightly less than perfect for this observation. (I didn't want to illustrate this as if the star was being occulted by its planet because that would add another complication. I think you see what I mean.)
Notice that the star changes color as it's shifted back and forth. In a real situation, this much shifting would not occur. The orbital velocity required to make a star change its color so much is far beyond the velocities available. Please accept this image as overly illustrative of the Doppler shift.
Besides, astronomers don't look for a star to change color! They measure the way absorption lines shift, as illustrated in the spectrum below this star. Only three bands are shown and, again, I've made the shift excessive, but you get the idea. In practice, astronomers would see only a very small shift in the absorption lines of the star's spectrum and would have dozens of lines to choose from. A Jupiter-sized planet orbiting pretty close to its star would shift the wavelengths of the star's spectrum by only one part in 10 million! (So a sodium absorption line at 5896 Angstroms would fluctuate between 5896.0005896 and 5895.9994104 Angstroms.)
This very small shifting will occur with clock-like precession determined by the orbital period of the invisible companion. Radial measurements by Doppler shifting is actually easier to measure (with the right instruments) than transverse motion involving astrometry. In the past few years astronomers have detected many invisible companions around a variety of stars and most of those observations have been made by careful measurements of the changes in radial velocity as measured by Doppler shifting. Sadly, Barnard's Star is not one of them. (However, you would expect that. If a planet orbited its star in such a way as to cause pertubations in its transverse motion, it would have the worst possible geometry to detect radial motion. Think about that and compare the images you have seen on this page to understand what I mean.)
Are any of those Doppler-shifting stars easy to see (unlike Barnard's Star)?
Sadly, no. (But next month I'll show you where some unconfirmed extrasolar systems may lie.)
Pegasus 51 is the most likely candidate as a star, other than our own, to have a planet. In October 1995 two astronomers at the Geneva Observatory in Switzerland, Michel Mayor and Didier Queloz, reported the first (uncontested and since then confirmed) planet around another star. Their calculations, all based upon Doppler shifting, indicate that it has a very large planet, about the size of Jupiter (or maybe half as small but no smaller). This planet has an orbital period of only 4.23 days which means it must be 0.2 Astronomical Units from the star. (Mercury is about 0.35 AUs from the Sun.) That would be one warm world - estimated to be about 1300oC! Peg 51, as this star is often called, is very similar to our Sun, type G2, but it's 45 light-years away making it below the limits of naked eye visibility.
Several other stars have been found to have a periodic Doppler shift. Their "invisible companions" are either brown dwarfs or big planets. In the near future astronomers will have access to more powerful equipment and should be able to measure smaller fluctuations in a star's motion and, from that, calculate the orbits of smaller "invisible companions" including those the size of the Earth!
But that's another story and the search for extrasolar planets is a specialized subject. To completely
appreciate this material requires a solid education in math and
physics but I am not trying to teach that kind of detail here.
Instead, I have given you a review of some of the methods astronomers
use to draw conclusions from their observations and, in the process, explained how we measure the motion of stars.
Next month I'll tell you about the universe and a wonderful branch of astronomy called "cosmology". (What a great way to finish the course! ) It's important that you understand distances, motion and AGNs (especially quasars) before you take on cosmology so make a point to study these subjects. Then, get outside and look at the universe!
See you next month.
Wishing you "Clear Skies".
Jamie (Dr Love)