# A On a Clear Day, at 6 ft tall, at the waters edge ....

1. Dec 22, 2015

### WallyTheInept

I can see, about 5 Km
Not bothered about Isobars or Kilojules.....
So, on that basis, if I am stood on the Earth, looking at a star that is shining.....not caring if it went out years ago.
How far away, can I see a star, shining, in the night sky, using just my eyes ?
On a clear, not foggy or misty night ?
In kilometers ?

2. Dec 22, 2015

### rootone

Stars hugely differ in luminosity, although they are all point sources as far as our eyes can tell.
Some red dwarf stars (by far the most numerous), are fairly close to Earth but invisible to the naked eye,
on the other hand some stars which are further away are visible because of they are very bright and large stars.

Oh, and by the way, forget about kilometers for this.
The very nearest visible star Proxima Centauri is 4.24 light year away
1 light year is about 9 trillion km.
Many visible stars are thousands of times more distant than that.

3. Dec 22, 2015

### davenn

from wiki
A light-year (abbreviation: ly), sometimes written light year,[3] is a unit of length used informally to express astronomical distances. It is approximately 9 trillion kilometres (or about 6 trillion miles).[note 1]

so now start looking at the distances to some of the more distant stars that can be seen naked eye from earth
and do the maths

is you use a program like Stellarium and pic on stars that are around magnitude 5 or brighter ( M4 is brighter than M5) the program will tell you the brightness and the lightyear distance to the star then convert to km's or miles

we don't use km or miles to other stars and galaxies etc ... the numbers are just too big

Dave

4. Dec 22, 2015

### davenn

that didn't really answer the Q

If he picks on stars that are around the M5 - M6 magnitude ( avg naked eye visual limit) then he can search for ones in that range and Stellarium will show their distances.
I suspect the OP didn't really realise the vast distances to the stars

Dave

5. Dec 23, 2015

### WallyTheInept

Thank you all for the prompt response, Yes I can see its going to be a large number, one of those questions that got put out at the marina bar.
Cheers All.

6. Dec 23, 2015

### Janus

Staff Emeritus
A bit of research seems to indicate that the Star Mu Cephei at 4300-9300 ly is likely the most distant naked-eye visible star. YOu might still see the star V76L-Cas as being listed at 16000 ly, but apparently newer measurements have put it much closer.

7. Dec 27, 2015

### _PJ_

Whilst I appreciate the question was specific to a star, and of course, the OP may want only to consider actual, individually resolvable stars - It might be interesting purely as point of interest if nothing else, that the Andromeda Galaxy is visible with the naked eye, and (whilst it obviously has comparative size) is approximately 2 500 000 ly away!

8. Dec 27, 2015

### jambaugh

This reminds me of a (locally famous at the time) trial that my father described to me once. The defense lawyer was cross examining the prosecution's eye witness who wore a rather strong prescription of glasses and asked him, "So Mr. [Smith], how far can you see with those glasses?" to which the witness replied! "I can see the Moon! How far is that?"

9. Dec 29, 2015

### jambaugh

BTW, the defendant was convicted.

10. Jan 2, 2016

### tfr000

Not unlike the oft-asked question, "How far can you see with your telescope?"
"Well, I've seen 3C273..."

11. Jun 5, 2016

### Alan McIntire

Different stars have different magnitudes, different people have different visual accuity
Arbitrarily assuming that magnitude 6 is the dimmest star that one can see with the naked eye,

http://astrobob.areavoices.com/2010/10/02/how-to-see-the-farthest-thing-you-can-see/

"The farthest star we can see with our naked eye is V762 Cas in Cassiopeia at 16,308 light-years away. Its brightness is magnitude 5.8 or just above the 6th magnitude limit. "

12. Jun 5, 2016

### davenn

D

13. Jun 5, 2016

### Staff: Mentor

In 1054, a star was seen from 6,500 light years away - during daylight - when it went supernova. Now we call it the Crab Nebula.

14. Jun 5, 2016

### davenn

well, if you really want to go there .....

in 1987, SN1987A in the LMC shone brightly naked eye for a month or so .... 170,000 light years away

Dave

15. Jun 5, 2016

### Staff: Mentor

Yeah, I did - but I was too lazy to search long enough for one that was outside our galaxy (and didn't remember any offhand).

16. Jun 5, 2016

### davenn

didn't have to do searches for that example .... one of the hilites of my astronomy hobby
Did photo's of that one a way back then ... real photo's ... on film ... none of this new fangled digital stuff

even got one shot of the supernova and comet Wilson in the same frame

17. Jun 7, 2016

### WallyTheInept

Wow, Thanks all.
This got rather complicated, rather quickly.
Thanks for the responses, Im caught up in life situations at the mo, so I am not here much, these days.
Thanks again
TTFN