19 replies to this topic

[Bluefish]

I was under the impression, most ALL the stars one can see under a class 1 sky were within 2,000 or so light years with some exceptions.  In other words, the only stars we see are the ones real close to us no matter its size.

 

What I don't get is, VY Canus Major, the craziest hypergiant is supposedly 5,000 light years away, but we cant see it with naked eye.  Yet V762 Cas is about 16,000 light years away and can be seen with naked eye?  Betelgeuse is only 650 light years away.  I’m confused.

 

Can you kind folk help me out with naked eye star distances?

 

I understand there are many variables with stars - volume, size, mass, luminosity, temp, etc..  For instance a volume may be millions of times bigger than our Sun on a given star, but only a hundred times the mass of our Sun.  But I still don’t understand the distance variations under naked eye observation.

[Keith Rivich]

You are correct in your impression. Most visible stars are within 2000 light years. And number of visible stars (not including the clouds of the Milky Way) is a lot less then most people think. Only around 10,000 stars brighter then 7th magnitude. 

 

The only variable that matters for visual detection is brightness. Size doesn't matter. Distance doesn't matter. Volume doesn't matter. All that matters is that the star is brighter then about mag 7 for a B1 site.  

[bobzeq25]

It's kinda bright.

 

The absolute magnitude of Sirius is 1.4.  The absolute magnitude of V762 is about -7 (about because its distance is somewhat uncertain).

 

Betelgeuse is somewhat comparable at -5.

 

Note that magnitude is a logarithmic scale, V762 is thousands of times brighter than Sirius.  Put it at the distance of Sirius, it would be light pollution.  It would cast a shadow.

 

 You say the word "stars" and your intuition thinks they're comparable things.  Not hardly.  You can't even see the Sun in the picture below.  You need to click on the picture to zoom in.  And then you can see it.  Barely.

 

Famous saying.  "The Universe is not only stranger than we imagine, it's stranger than we CAN imagine."

 

Edited by bobzeq25, 30 December 2023 - 09:00 PM.

[therealdmt]

Think more like, very, very roughly, 10,000 light years for the radius of the volume of individual, resolved stars that can be seen naked eye (you gave an example of a star roughly 16,000 light years distant), but with the huge majority of such stars one can see naked eye being much closer than that. A few thousand light years, but as you have already pointed out with examples, there are certainly stars that can be seen beyond 2,000 light years. Also, bear in mind that some people can see much dimmer stars than others, some skies are much darker and more transparent than others, and the error bars for distance measurements grow with distance (i.e., it’s difficult to precisely determine just how far a far away star is). Accordingly, the farthest star that can be seen will vary significantly depending on observer and conditions, and beyond that, it is problematic to say with certainty just how far away these stars are (though the GAIA mission is in the course of greatly improving our understanding of these distances). 

 

Interestingly to me, the very closest star to us after the Sun, the red dwarf Proxima Centauri, is so small and dim that it cannot be seen naked eye. Then there are its neighbors/partners of Alpha Centauri, a close pair of sun-like stars that are very bright in southern hemisphere skies, then a number of red dwarfs (including perhaps most famously, Bernard’s Star and Wolf 359) that cannot be seen at all naked eye (though perhaps a sharp-eyed observer could detect the mag 7.5 red dwarf Lalande 21185 under ideal conditions?), and then Sirius, a star inherently brighter than the Sun but not one of the inherently brightest stars, but which is seen as the very brightest star in our night skies due to its relative proximity. 
 

Another interesting-to-me case is the stars of the Summer Triangle, three of the brightest stars in northern hemisphere skies: Vega (approx. mag 0) , Altair (mag 0.8) and Deneb (mag 1.3). Vega is about 25 light years away, but the slightly dimmer Altair is almost twice as close at about 17 light years away. And then Deneb, the least bright of the three but still similarly bright to the casual glance, is over 2,000 light years away. There are a few significantly different distances that come up for Deneb, but anyway, these three famous stars are similarly bright, but one of them, Deneb, is much, much farther than the others.

 

Why? Some stars are just inherently (absolute magnitude) much, much, much bigger and brighter than others. You may have seen this size comparison chart before, but it’s always a bit mind blowing to look over again: https://en.wikipedia...s.jpg#filelinks

 (after clicking the link, you might have to scroll all the way up to see the image — testing my link, it seems to keep taking me directly to halfway down the page... Anyway, once you get to it, you can click on the image to see it bigger for a better effect)

[TOMDEY]

Nearly all of the couple of thousand brightest stars are (as one would expect) relatively close, and also average out in nearly all directions. Same can be said for nebulae and galaxies. Confirms the two reasonable presumptions of basic observational astronomy... that, to 1st order, our universe is both homogeneous and isotropic.    Tom

[Tony Flanders]

What I don't get is, VY Canus Major, the craziest hypergiant is supposedly 5,000 light years away, but we cant see it with naked eye.  Yet V762 Cas is about 16,000 light years away and can be seen with naked eye? Betelgeuse is only 650 light years away.  I’m confused.

When I did a Google search on "V762 Cas" my first hit was this website. Here are some quotes from that article:
 

V762 Cassiopeiae is 2,764.10 light-years away from us.
...
Furthest Star Visible with the Naked Eye
 
V762 Cassiopeiae was at one time regarded as being the furthest star you can see with the naked eye. Any further you'll need either a binocular or telescope. In 2007, the parallax, the figure that is used to calculate the distance became larger and therefore its distance was calculated as being much closer.

 
Prior to HIPPARCOS, errors in measuring parallax were on the order of 10 milliarcseconds or more, meaning that parallax was essentially useless for stars more than 100 parsecs (300 light-years) distant.

 

The typical error of HIPPARCOS parallaxes is thought to be about 1 milliarcsecond, meaning that HIPPARCOS star distances are mostly quite reliable out to 1,000 light-years, and fairly reliable somewhat farther than that. Even so, quite a number of naked-eye stars are far enough so that distances based on HIPPARCOS are pretty much meaningless.

 

GAIA parallax uncertainty is thought to be better than 0.1 milliarcsecond for most stars brighter than about magnitude 15, so it's now possible to make reasonable statistical studies for stars to 10,000 light-years or farther. Unfortunately stars brighter than about magnitude 4 saturate GAIA's sensors, meaning that for the very brightest stars HIPPARCOS is still better than GAIA.

 

Betelgeuse is an interesting special case; it's impossible to measure its distance with any certainty because the star is too big, and its surface is unevenly bright. Its apparent size is around 50 milliarcseconds, which is considerably bigger than its parallax. So when attempting to measure its parallax you're basically comparing two large disks that overlap for most of their diameter. And unfortunately it's impossible to tell for sure where the centers of those disks are, because Betelgeuse has huge star spots that make its surface quite irregular.

 

Regardless, Betelgeuse is certainly quite close to us as supergiants go. The reason it doesn't appear even brighter than it actually does is that it's also pretty puny as supergiants go.

[Tony Flanders]

Betelgeuse is an interesting special case; it's impossible to measure its distance with any certainty because the star ((appears)) ... considerably bigger than its parallax.

It occurs to me that if you think about the geometry of the situation, a star's apparent size is bigger than its parallax for any star whose diameter is greater than Earth's orbit. Which includes all red supergiants and perhaps even some red giants.

[yuzameh]

I was under the impression, most ALL the stars one can see under a class 1 sky were within 2,000 or so light years with some exceptions.  In other words, the only stars we see are the ones real close to us no matter its size.

 

What I don't get is, VY Canus Major, the craziest hypergiant is supposedly 5,000 light years away, but we cant see it with naked eye.  Yet V762 Cas is about 16,000 light years away and can be seen with naked eye?  Betelgeuse is only 650 light years away.  I’m confused.

 

Can you kind folk help me out with naked eye star distances?

 

I understand there are many variables with stars - volume, size, mass, luminosity, temp, etc..  For instance a volume may be millions of times bigger than our Sun on a given star, but only a hundred times the mass of our Sun.  But I still don’t understand the distance variations under naked eye observation.

Depends where you are getting your numbers from, according to gaia parallax vy cma is three times further away than v762 cas.  And let's not forget interstellar absorption/extinction for the respective lines of sight.

[Bluefish]

Okay, much of this is over my head.  Just cant understand why we cant see VY canus major?  The hyperest of hyper giants, supposedly biggest star in the galaxy at only 5000 LY.  

 

When one says only 10,000 stars are visible with naked eye under best conditions.  Is that from one viewing location or all over the world?

[Keith Rivich]

Okay, much of this is over my head.  Just cant understand why we cant see VY canus major?  The hyperest of hyper giants, supposedly biggest star in the galaxy at only 5000 LY.  

 

When one says only 10,000 stars are visible with naked eye under best conditions.  Is that from one viewing location or all over the world?

VY Canis Majoris is a variable star that can range from  ~Vmag 7 to ~Vmag 9. The bulk of its light is emitted in the infrared part of the spectrum due to it being embedded in a huge molecular cloud, Sharpless 2-310. That reason alone is why you can't see it naked eye. 

[mikemarotta]

Another interesting-to-me case is the stars of the Summer Triangle, three of the brightest stars in northern hemisphere skies: Vega (approx. mag 0) , Altair (mag 0.8) and Deneb (mag 1.3). Vega is about 25 light years away, but the slightly dimmer Altair is almost twice as close at about 17 light years away. And then Deneb, the least bright of the three but still similarly bright to the casual glance, is over 2,000 light years away. 

 

Thanks! That can help for outreach when talking to people about brightnesses and distances. The Andromeda Galaxy is about 2.5 million light years away and contains upward of perhaps a trillion stars. Those numbers are difficult to put into everyday context. But Deneb - one star; right there - is about 2000 light years away. We see it as it was at the last high-water mark of the Roman Empire. With some exceptions, it is the farthest that you can see naked eye.

 

Clear Skies,

Mike M.

[bobzeq25]

VY Canis Majoris is a variable star that can range from  ~Vmag 7 to ~Vmag 9. The bulk of its light is emitted in the infrared part of the spectrum due to it being embedded in a huge molecular cloud, Sharpless 2-310. That reason alone is why you can't see it naked eye. 

And, note that it's a close thing with V762.  Apparent magnitude 6.5 versus 6.1.

 

Bottom line, you can't go by your intuition.  This is science, and you need to know the numbers.

[Tony Flanders]

VY Canis Majoris is a variable star that can range from  ~Vmag 7 to ~Vmag 9. The bulk of its light is emitted in the infrared part of the spectrum due to it being embedded in a huge molecular cloud, Sharpless 2-310. That reason alone is why you can't see it naked eye.

Right. It's easy to forget about extinction. We sort of assume that if you have two identical stars, one twice as far as the other, that the more distant star will be exactly 1/4 as bright. But that assumes there's no dust in the way to block the view, which in practice is often not true. Especially in the case of hypermassive stars, which eject vast quantities of dust.

For instance Eta Carinae, a strong candidate for being the most luminous star around, is completely invisible in normal light; all we can see is the cocoon of dust around it.

Likewise, we cannot see the core of our own galaxy because it's completely blocked by dust along our line of sight. Presumably without that dust in the way it would be readily visible even from the middle of major cities, and conceivably even in broad daylight.

[JohnTMN]

Okay, much of this is over my head.

It's easy, distance has nothing to do with brightness.

Your neighbors porch light. Is it a 20 watt red one? Or a 300 watt clear? Same distance, different brightness.

Point is, Distance, Brightness, Color, all apply to our viewing and related to what we can see.

But distance, isn't the only factor.

Some fires burn hot,(like a blast furnace) some are only embers left from last nights bon fire. 

Edited by JohnTMN, 01 January 2024 - 04:28 AM.

[JOEinCO]

When one says only 10,000 stars are visible with naked eye under best conditions.  Is that from one viewing location or all over the world?

 

I think you misunderstood Keith. 

 

There are about 10,000 stars TOTAL, in every direction, that are magnitude 7 or brighter as seen from our part of the cosmos.

 

Magnitude 7 is getting pretty dim for naked eye. You need decent eyes, very complete dark adaptation, and you need a pretty dark observing site. And even then, on average, only half are above the horizon at any given time because that 10,000 number is the "total, every direction" number.

[mikemarotta]

Magnitude 7 is getting pretty dim for naked eye. You need decent eyes, very complete dark adaptation, and you need a pretty dark observing site. And even then, on average, only half are above the horizon at any given time because that 10,000 number is the "total, every direction" number.

 

Ptolemy’s Almagest listed 1022 stars and catalogues by Abd al-Rahman al-Sufi (10th century CE) and Ulugh Beg (1394-1449 CE) extended that to 1437 stars. In 1782 Johann Elbert Bode published the positions of 5,058 stars down to the 7th magnitude. -- Knobel, E. B. (1877). “The Chronology of Star Catalogues,” Memoirs of the Royal Astronomical Society, Vol. 43.

 

Telescope limiting magnitudes are variously given by different authorities such as the RASC as <something> + 5 log D where D is the aperture in millimeters. Usually 6 or 6.5 is the naked-eye baseline.

 

Thanks,

Mike M.

[Tony Flanders]

Magnitude 7 is getting pretty dim for naked eye. You need decent eyes, very complete dark adaptation, and you need a pretty dark observing site. And even then, on average, only half are above the horizon at any given time because that 10,000 number is the "total, every direction" number.

Moreover, a large fraction of all the stars that are technically above the horizon at any time are very low in the sky, where they're subject to massive dimming by the atmosphere. As I remember, at any given moment you can only see about 1/3 of all the stars that would be visible if they were directly overhead. And that assumes zero light pollution and perfectly clear air.

 

On the other hand, many people have reported seeing stars quite a bit fainter than magnitude 7.0 from really good sites, like up on the Altiplano that straddles the Bolivia/Chile/Argentine border.

[Ben the Ignorant]

I has nothing to do with size. All stars, even the closest and/or the biggest ones, are sizeless dots to the naked eye because of distance. Some stars, those with strong apparent brightness, look bigger because light smears a little around the dot's core. All stars produce this effect, but only the brightest make a smear bright enough to be seen. Imagine two photos of the Pleiades, for example, one is exposed for a short time, the faint stars look small. The same setup takes a longer exposure, and the faint stars look big.

 

The basic thing you're missing is, some stars like Deneb are 60.000 times brighter than the Sun, and the faintest red dwarves emit only 1/10.000 of the Sun's output. Yeah, that's a 600.000.000 range.

[birger]

Ptolemy’s Almagest listed 1022 stars and catalogues by Abd al-Rahman al-Sufi (10th century CE) and Ulugh Beg (1394-1449 CE) extended that to 1437 stars. In 1782 Johann Elbert Bode published the positions of 5,058 stars down to the 7th magnitude. -- Knobel, E. B. (1877). “The Chronology of Star Catalogues,” Memoirs of the Royal Astronomical Society, Vol. 43.

 

Telescope limiting magnitudes are variously given by different authorities such as the RASC as <something> + 5 log D where D is the aperture in millimeters. Usually 6 or 6.5 is the naked-eye baseline.

 

Thanks,

Mike M.

I looked briefly at Johann Bayer's Uranometria from 1603 (just before the telescope was invented). I'm not sure if Bayer himself watched the stars and placed them on a map, or if he just compiled a catalog based on other people's observations, but the faintest star I found in the book (specifically k Virginis, also known as 44 Virginis) is about +5.8.

Edited by birger, 10 January 2024 - 01:09 PM.

[Tony Flanders]

I looked briefly at Johann Bayer's Uranometria from 1603 (just before the telescope was invented). I'm not sure if Bayer himself watched the stars and placed them on a map, or if he just compiled a catalog based on other people's observations, but the faintest star I found in the book (specifically k Virginis, also known as 44 Virginis) is about +5.8.

Uranometria was based on the star catalog of Tycho Brahe except for the map of the far-southern sky, which Brahe could not see from Denmark. Brahe's prototype was the star catalog in the Ptolemy's Almagest, which was at that time (and had been for 1,500 years) the standard astronomy textbook. Both Ptolemy and Brahe missed quite a number of 4th- and 5th-magnitude stars.

 

I would have expected the faintest star in Uranometria to be fainter than mag 6.0, but I haven't attempted to collate the data myself. Mind you, nobody ever claimed that either Ptolemy's catalog or Brahe's was complete. Brahe was a great observer; he certainly knew that there were plenty of naked-eye stars that he didn't include in his catalog. But measuring the position of a star that's only visible with averted vision is easier said than done.