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Okay, bear with me. In layman's terms:

Еstimates vary, but on a clear night, away from big cities and their "light pollution," you'll see about 5000 stars up in the sky, give or take. Even though all of them are in the Milky Way, they're not equidistant from Earth: they revolve around the galactic center at different orbits. Some are in our immediate neighborhood while others hang out as far as 1000 light year away.

5000 stars may not be a great number by astronomical standards, but it's still pretty impressive, wouldn't you say?

Some stars are larger, and others smaller. It stands to reason that some of them might be "hiding" behind others. Because they orbit the galactic center at different speeds, using different orbits, wouldn't it follow that from time to time, over the centuries, decades, or even years (months?) a "new" star would emerge from behind a well-documented one? Shouldn't this be happening often; or are they so sparsely distributed around the galaxy that no "new" stars have appeared out there over the past three thousand years or so?

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  • $\begingroup$ Look up proper motion of stars. en.wikipedia.org/wiki/Proper_motion#History Stars move across the sky very slowly. Halley was the first to nice that 3 stars had drifted more than 1/2 of 1 degree compared to written records from about 1850 years earlier. $\endgroup$
    – userLTK
    Jan 3 '16 at 0:42
  • $\begingroup$ @userLTK: Which is why I said, "Over the past three thousand years or so." $\endgroup$
    – Ricky
    Jan 3 '16 at 0:46
  • $\begingroup$ We haven't been systematically observing and cataloging stars for very long. It's more like "no 'new' stars have appeared out there over the past few hundred years". $\endgroup$
    – HDE 226868
    Jan 3 '16 at 1:11
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    $\begingroup$ @HDE226868: from Wikipedia: "Proper motion was suspected by early astronomers (according to Macrobius, AD 400) but proof was provided in 1718 by Edmund Halley, who noticed that Sirius, Arcturus and Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly 1850 years earlier." So, no, not merely the past few hundred years. $\endgroup$
    – Ricky
    Jan 3 '16 at 1:15
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    $\begingroup$ @Ricky I mean that it's only in recent history that we can identify a significant number of stars and consider them all unique. Hipparchus observed a few of the brightest stars he could see. He could not observe and distinguish most of the others with the necessary accuracy. Ptolemy, for example, only wrote down about 1,000 in his catalogue. $\endgroup$
    – HDE 226868
    Jan 3 '16 at 1:19
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To the naked eye, the answer is almost certainly no because of the enormously slow movement of stars across the sky and because 5,000 stars may be a lot but only a tiny percentage of the sky is covered by visible stars.

To Hubble, which can see perhaps tens of millions of stars, maybe more, the link here, has a picture of two stars that are approaching crossing each other's path from our point of view. With a big enough telescope it probably happens from time to time, though I wouldn't want to try to calculate how often, but to the naked eye, I'm comfortable saying no, in fact, it was often assumed that stars didn't move and were fixed in the sky (contrary to what Macrobius said). That was the popular point of view prior to Halley's observation.

There was also Tycho Brahe's "De Nova Stella" or "new star" which we now know to be a super-nova, and that was quite the surprise at the time. Nobody thought a new star could appear because they thought the stars were fixed and permanent, but that appearance wasn't by the method you suggest.

Consider how small stars are from our point of view. Alpha Centauri A, the larger one, it's about 1.7 million KM across and it's about 4.3 light years away, or, 41 trillion KM. It's diameter is 23 million times smaller than it's distance from us. That's the equivalent of looking at a golf ball from nearly 200 miles away. Now if you scatter 5000 golf balls each 200 miles away across the sky and you let them move around very very slowly, how often do you think one golf ball passes in-front of another? Not very often. Granted, that's not quite right as the atmosphere spreads stars out a bit so each golf ball is smudged to maybe the size of a basketball, but they almost never pass infront of one another, at least, not if we only take into account 5,000 visible stars.

Now, binary stars, it happens more often if they are lined up right, then they can pass infront of each other and this has certainly been observed by telescope but not to the Naked Eye, we can't visibly tell that Alpha Centauri is 2 stars (3 with the more distant Proxima but that can't be seen by the eye). They are on average about a billion miles apart but that can't be seen by the naked eye. It was observed by telescope in 1689.

There simply aren't enough visible stars (and taking HDE's point that most of the 5,000 visible stars weren't cataloged until recently), there's essentially zero chance that it was ever observed that a star appeared "new" by passing from behind another star.

Using Hubble, it can happen, but not to human sight.

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  • $\begingroup$ " ... the link I provided above ..." -- Please include that link in your answer. $\endgroup$ Jan 4 '16 at 16:31
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    $\begingroup$ FYI .. We haven't even discovered all of the M stars out to 100 light years yet. I'm sure that Hubble could aim at a new field of view and discover more every time. As far as cataloging, we have done about a billion stars now and there are several hundred billion in the Milky way alone. $\endgroup$ Jan 6 '16 at 4:27
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Once it was discovered about 300 years ago that stars have "proper motion", and appear to move very slowly over the celestial sphere in random difections and random apparent speeds, it was realized that the stars were all moving in three dimensional space.

And if all stars have the same actual velocity in space, or velocities within a relatively narrow range, the stars with the largest proper motion are likely to be nearer to Earth than stars with lower proper motion.

So astronomers naturally tended to try to measure the parallexs of the stars with the largest known proper motions first, since they were probably among the closest stars to Earth. Astronomers also realized that if the different stars all had the same luminosity, the ones which appeared brighter from Earth would have to be closer.

Since the closer a star was to Earth, the larger its parallax would be, and this the easier it would be to measure the star's distance, astronomers concentrated their parallax measuring efforts on the stars which appeared brightest as seen from Earth, and the stars, however dim they looked from Earth, which had the largest proper motion.

The first distance measurements to stars were made in the 1830s, and tens hundreds, and thousands more were made in the next century. By about 1950, attempts had been made to measure almost all of the stars which appear visible to the naked eye from Earth. Many turned out to be too far away for their distances to be measured, and more or less accurate distances to the others were measured. And attempts were made, often successfull, to measure the distances to thousands of stars with large proper motions.

The Hipparocos satellite from 1989 to 1993, and the Gaia space observatory from 2013 toc. 2022, have made many percise astrometric observations of many mthousands andmilions of stars. For example, they have measured the proper motions of millions of stars far more accurately than before. So astronomers might be rushing to measure the parallaxes of all the high propermotion stars they detected.

Except that Hipparcos and Gaia also measured the parallaxes, and thus the distances, of the stars they observed, to much greater accuracy than ever before.

Some astronomers have had projects to discover all the stars closest to Earth and have searched for stars which might be nearby to measure their parallaxes.

Stars with the same spectral class can vary greatly in their luminosities depending on whether they are supergiants, bright giants, giants, sub giants, or main sequence stars. And apparently the spectra of stars can show which luminosity class they belong to.

So astronomers searching for neaby stars would pick luminosity class V stars, main sequence stars, to find the parallaxes of, because they would be much less luminous that giant stars with the same apparent brightness, and so would be much nearer.

So discoveries of stars that are really close to the Solar system has slowed down a lot as the lists become more and more complete.

The 1991 third Gleise catalog of stars within 25 parsecs (84.64 light years) of the Sun includes 3,803 stars. A sphere with a radius of 25 parsecs has a volume of 65,449.8469 cubic parsecs. So the stars in the catalog have a density of about 0.0581055 stars per cupic parsec.

Wikipedia's List of nearest stars and brown dwarfs lists stars and brown dwarfs within 5 parsecs of the Sun. That list has a volume of 523.598776 cubic parsecs. Omitting 11 brown dwarfs, and including the Sun, there are 65 stars in that radius. So the stars in that list have a density of 0.1241408 stars per cubic parsec.

However, the individual stars in the list are grouped in star sysems containing 1, 2, or 3 stars each. There are 50 star systems in the list, so the star systems have a density of 0.0954946 stars per cubic parsec.

I am not certain whether the Gleise catalog lists individual stars or star systems. So I don't know if the stellar density of listed stars in the Gleise catalog is 0.4680612 or 0.6084689 the density of stars in the list of nearby stars.

Either way, it indicates that the percentage of stars whose distances have been measured decreees with distances of just tens of parsecs.

There is another class of astronomical objects sort of similar to stars. Tthey are called brown dwarfs, intermediate in mass between planets and stars. Since brown dwarfs have only limited fusion during parts of their lifetimes, they are very cool, compared to the typcial star, and emitt almost all their radiation in the infrared bands.

In recent decades many brown dwarfs have been discovered. The closest examples are Luhman 16A & Luhman 16B, and WISE 0855-0744, (both discovered in 2013), at distances farther from the Sun than Barnard's Star, the second closest star system, and closer to the Sun than Wolf 359, the third closest star system. There are a total of 11 brown dwarfs within 5 parsecs.

So probably some more brown dwarfs close to the Sun will be discovered.

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  • $\begingroup$ +1. Yet 0.09 stars per cubic centimetre is a lot. Ymmd😂 $\endgroup$ 2 days ago
  • $\begingroup$ @planetmaker corrected. $\endgroup$ 2 days ago
  • $\begingroup$ Well, yeah, but there's supposed to be BILLIONS of them! $\endgroup$
    – Ricky
    yesterday

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