- As it is north of the celestial equator, Vega will be above our horizon for more than twelve hours each day. In fact, Vega remains above the horizon for about 18 hours each day. As this star is positioned so far north, it is a "Vegan star," meaning that it is visible for at least part of the night every night of the year even though it is not circumpolar. Actually, Vegan stars are named for the star Vega. For instance, one can see Vega low along the northern horizon around early evening in February.
- Vega rises about four minutes earlier each day due to Earth's revolutionary motion around the Sun. Vega will therefore rise about half an hour earlier each week; two hours earlier each month.
Next, we know that Vega's parallax angle is 0.130" (arc-seconds).
How can we determine its distance? We divide one by 0.130 = 7.69 parsecs.
One parsec equals 3.23 light years, so Vega's distance is approximately 25 light years.
Its apparent magnitude is 0.26, making it the fifth brightest star in the sky.
Knowing it distance and apparent magnitude, we can use the distance modulus equation to determine its absolute magnitude. We remember that absolute magnitude equals the star's apparent magnitude were it at a distance of 10 parsecs. Vega's absolute magnitude equals 0.582. Recall that the Sun's absolute magnitude equals 4.83.
With an absolute magnitude of 0.582, Vega is 40 times more luminous than the Sun.
- Next, by scrutinizing Vega's spectrum (the light emission), we can determine that Vega is an A0 V star, toward the hotter end of the star sequence. We remember that sequence as O B A F G K M, with O's being the hottest stars; M's the "coolest." Luminosity class V means it is a dwarf star and belongs along the main sequence. At this moment we should tack Vega on the H-R Diagram:
The H-R Diagram will yield even more information about this bright Summer Triangle star. By Vega's placement on the H-R Diagram, we see that its radius is greater than the Sun. (2.4 times greater.) Also, it is more massive than the Sun (2.1 times greater) and will therefore spend less time on the main sequence (less than one billion years.)
So, with just a little information:
- apparent magnitude
- parallax angle
- spectral type
- declination
we were able to determine
- visibility region; zenith declination; circumpolar region
- distance
- absolute magnitude
- luminosity
- radius
- mass
- lifetime on the main sequence
We leave it as an exercise for you to glean as much information as you can about Altair based on the following information:
- apparent magnitude: 0.76
- parallax angle: 0.194"
- spectral type: A7 V
- declination: almost +9
We will soon be moving on from the stars. Before we did, however, we thought it would be instructive to put all these facets together to show how astronomy enables us to ascertain so much about a star that is so distant.