<p>We would have 
<span class="math-container">$$\tau = d/0.99c - d/c,$$</span>
of warning, where <span class="math-container">$d$</span> is the distance to the star when we first detect its light and <span class="math-container">$c$</span> is the speed of light.</p>

<p>Would we see it - 
yes indeed. Since
<span class="math-container">$$\lambda_{\rm obs} = \lambda_0 \left( \frac{1 - v/c}{1+ v/c}\right)^{1/2},$$</span>
the light from the star would be blueshifted by a factor of <strong>14</strong>, so for a solar type star, the peak of its emission would be shifted into the ultraviolet.</p>

<p>However, it will still be much brighter than a stationary star at all wavelengths. If we assume the star emits roughly blackbody radiation, then it will look like a blackbody at 14 times the temperature, with an integrated luminosity that is <span class="math-container">$14^4 = 38,000$</span> times higher!</p>

<p>See the results/derivations at <a href="https://en.m.wikipedia.org/wiki/Relativistic_Doppler_effect" rel="noreferrer">https://en.m.wikipedia.org/wiki/Relativistic_Doppler_effect</a></p>

CC BY-SA 4.0
2019-11-19T16:48:55.333Z
So if it travelled at the speed of light wouldn't it have infinite mass and then turn into a black hole?? :-)
MiguelH
CC BY-SA 4.0
2019-11-19T16:53:07.697Z
@MiguelH It cannot travel "at the speed of light". The question asks what it would look like if it travelled at 99% the speed of light.
ProfRob
CC BY-SA 4.0
2019-11-19T16:54:15.780Z
@uhoh  "It will still be much brighter than a stationary star at all wavelengths".  ??
ProfRob
CC BY-SA 4.0
2019-11-19T16:58:05.357Z
@RobJeffries. Fair cop! Well then, almost infinite mass ...
MiguelH
CC BY-SA 4.0
2019-11-19T17:05:36.803Z
@MiguelH You are out of date. "Mass" is a relativistic invariant, such that $(mc^2)^2 = E^2 - p^2c^2$. What you mean is that its momentum is very large.
ProfRob
CC BY-SA 4.0
2019-11-19T22:12:42.993Z
@RobJeffries Yes indeed, I'm not sure how I missed that.
uhoh
CC BY-SA 4.0
2019-11-20T15:44:24.947Z
@RobJeffries. Thanks for pointing that out! I now understand a little more!
MiguelH
CC BY-SA 4.0
2019-11-17T19:47:39.133Z
2019-11-19T11:10:48.810Z
2019-11-19T11:10:48.810Z
ProfRob
<p>Processes energetic enough to accelerate a star to 99% of the speed of light are not known. A close interaction and ejection from a multiple black hole system could perhaps provide enough energy, but such close interaction with black holes would probably rip the star apart before it could reach such excessive speeds. Hypervelocity stars are known. They have speeds less than about 1% of the speed of light.</p>

<p>A star travelling through space at such speeds would also have problems.  It would be slamming into interstellar gas. I have no idea if it could even survive, or would it be eroded by the interaction with the thin gases in interstellar space.</p>

<p>A star at that speed would be significantly blue-shifted. This would make its spectrum very odd.  A small dim star could be visible, catalogued and observed to have a very odd spectrum at a distance of about several hundred light-years. It is likely that we would first observe such a star several hundred years before it got close to us. However such high speed objects probably don't exist.</p>

CC BY-SA 4.0
2019-11-16T22:40:32.437Z
That's incredible thank you :)
Jake Graham Arnold
CC BY-SA 4.0
2019-11-16T22:48:45.803Z
Despite the "problems" the poor star would be having, I'd still like to know the answer to the question; "...would we see it?" if it were traveling near the speed of light.
uhoh
CC BY-SA 4.0
2019-11-17T07:13:45.600Z
See paragraph 3:  Yes, but it would be blue shifted.
James K
CC BY-SA 4.0
2019-11-18T20:04:48.500Z
Carl Sagan [video](https://www.youtube.com/watch?v=-CIs3jOnfiM) on "what if the speed of light were slower".
Keith McClary
CC BY-SA 4.0
2019-11-19T22:05:23.810Z
How do you get a blueshift of ~2? It should be more like 14 at v = 0.99c.
pela
CC BY-SA 4.0
2019-11-16T16:26:01.693Z
2019-11-19T22:13:33.830Z
2019-11-19T22:13:33.830Z
James K
<p>Stars being flung from black holes in the news made me question if something were heading directly for us at the speed of light, would we even know?</p>

<p>Update: Since it's impossible for a star to travel at the speed of light, would we see it coming if it were traveling at 99% the speed of light? and if so, how much warning would we have? </p>

CC BY-SA 4.0
2019-11-16T13:40:46.280Z
Ah ok sorry I didn't know I'll update the question
Jake Graham Arnold
<star>
If a star were travelling at 99% of the speed of light, directly at Earth would we see it?
Jake Graham Arnold
# If a star were travelling at 99% of the speed of light, directly at Earth would we see it?

(Asked by: username_0 on 2019-11-16)

Stars being flung from black holes in the news made me question if something were heading directly for us at the speed of light, would we even know?

Update: Since it's impossible for a star to travel at the speed of light, would we see it coming if it were traveling at 99% the speed of light? and if so, how much warning would we have?

**Tags:** star
> **Comments:**
> **username_0**: Ah ok sorry I didn't know I'll update the question  

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## 2 answers

### Answer by username_4 on 2019-11-16. Score: 8 | Accepted answer

Processes energetic enough to accelerate a star to 99% of the speed of light are not known. A close interaction and ejection from a multiple black hole system could perhaps provide enough energy, but such close interaction with black holes would probably rip the star apart before it could reach such excessive speeds. Hypervelocity stars are known. They have speeds less than about 1% of the speed of light.

A star travelling through space at such speeds would also have problems. It would be slamming into interstellar gas. I have no idea if it could even survive, or would it be eroded by the interaction with the thin gases in interstellar space.

A star at that speed would be significantly blue-shifted. This would make its spectrum very odd. A small dim star could be visible, catalogued and observed to have a very odd spectrum at a distance of about several hundred light-years. It is likely that we would first observe such a star several hundred years before it got close to us. However such high speed objects probably don't exist.

> **Comments:**
> **username_0**: That's incredible thank you :)  
> **username_3**: Despite the "problems" the poor star would be having, I'd still like to know the answer to the question; "...would we see it?" if it were traveling near the speed of light.  
> **username_4**: See paragraph 3:  Yes, but it would be blue shifted.  
> **username_5**: Carl Sagan [video](https://www.youtube.com/watch?v=-CIs3jOnfiM) on "what if the speed of light were slower".  
> **username_6**: How do you get a blueshift of ~2? It should be more like 14 at v = 0.99c.  

---

### Answer by username_1 on 2019-11-17. Score: 8

We would have
$$\tau = d/0.99c - d/c,$$
of warning, where $d$ is the distance to the star when we first detect its light and $c$ is the speed of light.

Would we see it -
yes indeed. Since
$$\lambda\_{\rm obs} = \lambda\_0 \left( \frac{1 - v/c}{1+ v/c}\right)^{1/2},$$
the light from the star would be blueshifted by a factor of **14**, so for a solar type star, the peak of its emission would be shifted into the ultraviolet.

However, it will still be much brighter than a stationary star at all wavelengths. If we assume the star emits roughly blackbody radiation, then it will look like a blackbody at 14 times the temperature, with an integrated luminosity that is $14^4 = 38,000$ times higher!

See the results/derivations at <https://en.m.wikipedia.org/wiki/Relativistic_Doppler_effect>

> **Comments:**
> **username_2**: So if it travelled at the speed of light wouldn't it have infinite mass and then turn into a black hole?? :-)  
> **username_1**: @MiguelH It cannot travel "at the speed of light". The question asks what it would look like if it travelled at 99% the speed of light.  
> **username_1**: @uhoh  "It will still be much brighter than a stationary star at all wavelengths".  ??  
> **username_2**: @RobJeffries. Fair cop! Well then, almost infinite mass ...  
> **username_1**: @MiguelH You are out of date. "Mass" is a relativistic invariant, such that $(mc^2)^2 = E^2 - p^2c^2$. What you mean is that its momentum is very large.  
> **username_3**: @RobJeffries Yes indeed, I'm not sure how I missed that.  
> **username_2**: @RobJeffries. Thanks for pointing that out! I now understand a little more!  

---

