Stars In The Same Place

[Mr. Astronomy]

Notice that the stars stay in the same place for about 2 days, the reason for this is that Earth is not moving so fast and the stars are millions of miles away, so they look like they are in the same place, any way that's for the naked eye of a human. Now why does it change in a few days, the reason is because Earth is moving so you see different ones at different times. Remember any one like your grandmother if your travelling to another country she tells you to always remember that she is looking at the same star, you might think that she can see only other stars, but in fact she is looking at the same exact star.

[Mr. Astronomy]

I love to look at stars and wonder what they are called. :rolleyes:

[Guest MacPhee]

Remember any one like your grandmother if your travelling to another country she tells you to always remember that she is looking at the same star, you might think that she can see only other stars, but in fact she is looking at the same exact star.

 

I don't remember my grandmother providing any astronomical guidance like that! Did I miss out? I've only one memory of my grandmother, from a scientific angle. It's this - once, as a 10-year old child, I was ill in bed, suffering from mumps. Or chicken-pox, or some similar long-ago affliction.

 

Propped up on pillows, lying in my bed of pain, I was reading a book. It was a fascinating one, about the genesis of nuclear weapons. The title was "Birth Of The Bomb". My gran came in, said "What are you reading dear?" I showed her the book. She stared at it briefly. Then gave me a funny look. As if she thought I was a Martian. She stiffened in manner. The conversation ended, she withdrew from the room, and that was that.

 

Was that a typical experience? For science-minded kids?

[maddog]

Propped up on pillows, lying in my bed of pain, I was reading a book. It was a fascinating one, about the genesis of nuclear weapons. The title was "Birth Of The Bomb". My gran came in, said "What are you reading dear?" I showed her the book. She stared at it briefly. Then gave me a funny look. As if she thought I was a Martian. She stiffened in manner. The conversation ended, she withdrew from the room, and that was that.

Was that a typical experience? For science-minded kids?

I don't think I had anything of the kind regarding my grandparents either. I don't even think my grandmother looked up at the night sky (or she never said of anything about it). My memories of staying with my grandparents was when I was about 5 years of age and was before my precocious period of reading my brother's college astronomy textbooks while in jr high. Staying with my grandparents I heard of tall tails by my grandfather and riddles.

 

Now as to the original post. This is not an odd thing. The Earth moves around the sun. Through the year using Sidereal time instead Solar time (Sidereal is measure against the stars while Solar is measure against the sun), the stars will start to move slowly in this revolution. Coming back to the same time each year - the star position will be nearly the same. The small change of this is due to the Procession of the Equinox. This is simply the rotating of where the axis of rotation is pointing as the Earth moves around the sun. This rotational period is 26000 years approximately. This does not explain everything as their are a few more facets to deal with. This rotation is not smooth. It turns out that the axis of the rotation of the Earth bobs up and down as it processes. This is called Nutation. I forget the exact period of this though it is also measure in thousands of years. The other major factor though the least affecting is the rotation of the oblaticity of the Earth's orbit of the sun. Since the Earth is moving in an ellipse not a circular orbit (eccentricity <> 0). There is a semi-major axis along with a major axis. So this rotation is the rotation of one of these axes around the sun. It's period is about 86000 years. The last thing is the stars themselves are revolving around our galaxy. Though this takes 250 million years to make a complete circuit. One single year you can barely calculate any deviation from the norm on even procession (at least above noise). It is not until you have about 20 or 30 years of points in which you apply statistics to start to see any trends. Proper motion is the definition of a stars motion against the background start (ones farther away). This is an effect of these stars going around our galaxy (we are moving with them).

 

maddog

[Guest MacPhee]

Maddogs' excellent post makes clear, that many different movements influence the apparent position of a star. As we see it from Earth.

 

I wonder, whether all these different movements can combine, in the case of an individual star, to give this result - that the star appears to be coming directly towards the Earth? In a straight line. Such a star will stay in the same place in the sky. In the sense, that we won't see it deviating left or right, or up or down. Because it will be coming at us "head on", so to speak.

 

We will however see a steady increase in the star's brightness - as it gets nearer, it will increase in apparent magnitude.

 

Do we know of any stars which are staying in the same place, but getting steadily brighter?

[maddog]

Maddogs' excellent post makes clear, that many different movements influence the apparent position of a star. As we see it from Earth.

I wonder, whether all these different movements can combine, in the case of an individual star, to give this result - that the star appears to be coming directly towards the Earth? In a straight line. Such a star will stay in the same place in the sky. In the sense, that we won't see it deviating left or right, or up or down. Because it will be coming at us "head on", so to speak.

We will however see a steady increase in the star's brightness - as it gets nearer, it will increase in apparent magnitude.

A couple of things here. Yes these effects do combine. Just like general translations and rotations about an axis as exemplified by the dihedral group (rotations and translations in 3-space) do in general not commute. That is the order of the effect matters to the outcome. The only exception as to what does commute (I have found) is when one of these transformations are about an axis through the central object (Earth). These will commute.

 

Now as to the effect of a star moving directly towards us would be the example of leeward stars ahead of us in revolving around our galaxy. In addition to getting brighter their spectrum will show a doppler effect of blue-shifting, meaning their velocity is moving towards (relative to our reference frame). To look in the sky 90 degrees away from the constellation Sagitarius (center of our galaxy - approx) and in the plane of our galaxy. I would have to do some check to know exactly where - is where you would find such stars.

 

Do we know of any stars which are staying in the same place, but getting steadily brighter?

Steadily getting brighter would put the star away from equilibrium were it to be other than approaching. This usually results in a Nova or Supernova. Both cataclysmic events.

 

I hope this clarifies things.

 

maddog

Edited June 14, 2012 by maddog

[CraigD]

Welcome to hypography, Mr Astronomy! :) Please feel free to start a topic in the introductions forum to tell us something about yourself.

 

I always enjoy reading of someone your age (12, according to your profile) getting interested and thinking about astronomy. It’s an important age, because in the decade of your life, the decisions you make about your education and profession are likely to determine what kind of astronomer you’ll be: professional, dedicated armature, or no longer interested.

 

Remember any one like your grandmother if your travelling to another country she tells you to always remember that she is looking at the same star, you might think that she can see only other stars, but in fact she is looking at the same exact star.

While it’s true that people on widely separated place on Earth can see the same stars, for people at places with greatly different longitudes, there are stars one can see, but not the other. For instance, having never traveled south past about about 25° N, I’ve never directly seen the stars of the Southern Cross constellation, just as people in who have never traveled north past about 25° S have never seen Polaris.

 

I wonder, whether all these different movements can combine, in the case of an individual star, to give this result - that the star appears to be coming directly towards the Earth? In a straight line. Such a star will stay in the same place in the sky. In the sense, that we won't see it deviating left or right, or up or down. Because it will be coming at us "head on", so to speak.

 

We will however see a steady increase in the star's brightness - as it gets nearer, it will increase in apparent magnitude.

Steadily getting brighter would put the star away from equilibrium were it to be other than approaching. This usually results in a Nova or Supernova. Both cataclysmic events.

As maddog notes, a rapidly brightening star, as a rule, indicates that the star is rapidly becoming more luminous, as is the case with nova (a “super-charged” white dwarf star) or a supernova (the explosion of a much-larger-than-Sun-size star), not that it’s getting rapidly closer. Also, nearly all stars are variable, some more strongly than others, so a star observed to be rapidly brightening may be one of these kinds.

 

Astronomers have long known, and used to fairly precisely calculate the motion of nearby stars, a better way to measure the radial velocity (speed toward or away from us) of stars, by comparing their spectral shift. Because stars have distinct spectral lines the wavelengths of which are precisely know for a light source at rest relative to the observer, the deviation from this wavelength in an observed star’s spectrum can be used to calculate this speed very quickly.

 

Combined this with precise angular (proper motion) measurements, we know with fairly high precision the motion of all the nearby visible stars. You can find them in many public references, such as this Gliese catalog website.