# Talk:Betelgeuse

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## Question

Betelgeuse is something like 640 light-years away. So, if a spaceship travels only at the speed of light, it will take 640 years to get there. Suppose we assume that faster-than-light travel by a spaceship is possible (even though relativity theory says that is impossible). Suppose a voyage by a spaceship from Earth to Betelgeuse takes three days. How fast would the spaceship have to travel (how many times the speed of light) to reach Betelgeuse in three days? I'm writing a fiction novel and hope to get this detail right; I've been trying to work out the math but I am not a scientist.--Tomwsulcer (talk) 13:57, 5 November 2015 (UTC)

speed (assume "1" is exactly light speed, ie 186,000 miles per second)
time (in years)
distance (in light years)
speed x time = distance
speed x 640 years = 640 light years
speed x (3 days/365 days) = 640 light years
speed x .0082191 years = 640 light years
speed = 640 light years / .0082121
speed = 77867 times the speed of light

Is this right?--Tomwsulcer (talk) 13:57, 5 November 2015 (UTC)

Yes, although I would point out that 640 LY is only an approximate distance, give or take 20%, so “about eighty thousand” is about as precise as we can make it. (Of course in your fictional future the distance could be much better determined, but you could pick any exact figure between about 60,000 and 95,000 without contradicting present knowledge.)
Take note, that this page is for discussing improvements to the article, not the subject in general; please ask questions of this nature at the Reference Desk instead.—Odysseus1479 04:44, 6 November 2015 (UTC)
Thanks, , will take your excellent information into account. (I think that there is tangential relevance here, since this line of information means humans will never ever get anywhere near Betelgeuse unless we can fly faster-than-light, many multiples of faster-than-light, and what the scientific community believes is that f-t-l travel is impossible. So maybe something could be added to the article that it is seriously unlikely humans will never get close to Betelgeuse or, for that matter, any distant star?) From my experience in Wikipedia (since 2009) I've come to learn that the truly sharp folks (such as yourself) follow the talk pages of these technical articles so please forgive me for seeking out your expertise in this manner. I have not had much luck with the reference desk in the past but maybe I'll try it again next time something comes up.--Tomwsulcer (talk) 11:34, 6 November 2015 (UTC)

640 years = 640 years * 365.25 days/year = 233,760 days

233,760 days*lightspeed / 3 days = 77,920*lightspeed 172.75.129.94 (talk) 14:43, 23 December 2016 (UTC)

## Color Index B-V

The color index given by Hipparcos catalog is 1.5 whereas the source cited gives 1.85 . Since the source was published prior to Hipparcos launch I think the value should be updated. Or am I missing some point ? — Preceding unsigned comment added by 83.206.4.101 (talk) 10:13, 26 October 2016 (UTC)

The Hipparcos B-V index is derived from multiple sources and in this case has a very large statistical margin of error. I don't know exactly why, but appears to be a common problem for very bright stars in the Hipparcos & Tycho catalogue. It isn't hard to measure B-V and error ranges are typically hundredths or thousandths of a magnitude, not half a magnitude. Multiple other sources consistently give B-V for Betelgeuse within a tenth or two of +1.85. The Nicolet source used in the article is a weighted mean of multiple sources. A formal error range is not given but the result for Betelgeuse is considered of high quality. Additionally, a consistent U-B value is given in the same catalogue. Simbad gives photometry from an even older single-source catalogue, one of the inputs for Nicolet, but it also has B-V of +1.85. In short, the given value looks pretty reliable and the Hipparcos number is distinctly dodgy. Lithopsian (talk) 11:15, 26 October 2016 (UTC)

## Spinning, swallowing

J. Craig Wheeeler has suggested that c. 100,000 years ago. Betelgeuse swallowed a Sun-sized companion accounting for its unusually high spin-rate.

The theory has only just been published in the MNRAS (see also phys.org), I don't know what the criteria are for inclusion of new theores in astro-articles, but I note that we currently have rotational velocity at 5 km/sec, whereas they say 15 km/sec ("150 times faster than any plausible single star just rotating and doing its thing", whatever that means). --Hillbillyholiday talk 08:44, 20 December 2016 (UTC)

The value of 5 km/s is "v sin(i)", the projected rotational velocity. The actual equatorial rotational velocity depends on the value of i, the inclination. There is an assumption that it is 20°, but that comes with a big dose of salt. Given that, there is a reasonably well-accepted discrepancy between the very slow observed rotation rates of supergiants like Betelgeuse and the relatively fast rotation rates that give the best results for evolutionary models. The models may need tweaking (*do* need tweaking for other related things, which might solve this discrepancy), the observations might be wrong, there may be factors beyond those we already know that cause evolved stars to spin down, or funky things may have happened like swallowing a planet. No harm in mentioning a published peer-reviewed article, just don't give it Wikipedia:Undue weight (sources). Lithopsian (talk) 17:32, 20 December 2016 (UTC)
Thanks for explaining, Lithopsian. I don't think I should add anything – you are evidently far more qualified – I just wanted to note the new article. You say a "very slow observed rotation rate", yet the "150 times faster" quote seems to imply the opposite? I are confused. --Hillbillyholiday talk 00:26, 21 December 2016 (UTC)

The total lifetime from the start of the red supergiant phase to core collapse varies from about 300,000 years for a rotating 20 M☉ star, 550,000 years for a rotating 20 M☉ star, up to a million years for a non-rotating 15 M☉ star. It is not clear from this abstract nor this paper, both cited for this statement, what the correct figures are. Adriaan Joubert (talk) 21:27, 4 January 2017 (UTC)

This paper shows the models that this is all based on. Table 1 would be a useful reference. It contains values similar to those in the article, although not apparently exactly the same. Lithopsian (talk) 22:16, 4 January 2017 (UTC)
Thanks for that. I have changed the sentence in the article to read The total lifetime from the start of the red supergiant phase to core collapse varies from about 300,000 years for a rotating 25 M☉ star, 550,000 years for a rotating 20 M☉ star [...] so that the statement is now consistent with the reference you provided. Adriaan Joubert (talk) 10:48, 5 January 2017 (UTC)

## Gamma-ray burst etc.

Betelgeuse is not likely to produce a gamma-ray burst and is not close enough for its x-rays, ultraviolet radiation, or ejected material to cause significant effects on Earth. The citation for this statement is this paper which is proprietary; can someone with access to the paper verify this claim? Adriaan Joubert (talk) 11:13, 5 January 2017 (UTC)

arXiv preprint here. Lithopsian (talk) 15:18, 5 January 2017 (UTC)

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