THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 "Someday, in deep time, all the possibilities will come to fruition." THE DAILY ASTRONOMER Wednesday, January 13, 2016 Of Vega and Sirius TO HV Yesterday, we took much from Pandora's Jar. This jar contains the myriad astronomy questions we receive throughout the year. As often happens, answers to questions prompt other questions. For this reason a wit, whose identity I've fortunately forgotten, branded science an exercise in futility as it never solves any problem. It merely precipitates new problems by attempting to solve old ones. Well, bah humbug, you wet blanket. Look at it this way, science can still toast our crumpets without having to explain the nature of reality. The first question answered yesterday pertained to Sirius. The questioner asked if it was always going to be the night sky's brightest star. We explained that in about 200,000 years, Vega, presently the fifth brightest star in the night sky, will surpass Sirius in apparent brightness and therefore become the brightest night sky star. Even though it will still be farther away than Sirius, Vega will appear brighter because it is intrinsically brighter. After reading this answer, a subscriber asked "Why is Vega brighter than Sirius?" This question, which we should have addressed yesterday, we will answer now, as the DA hasn't devoted much time to explaining why stars differ in brightness so profoundly. We begin with the notion of "luminosity," defined as the amount of energy a star produces every second. A star's mass determines its luminosity. The more massive the star, the more luminous it will become. This mass-lumminosity relation is one of the cornerstones of astrophysics. Sirius is about two times more massive than the Sun and, consequently, about 25 times more luminous than the Sun. Vega is approximately 2.14 times more massive than the Sun. Though the mass difference between Sirius and Vega might seem slight, the luminosity is very mass sensitive.* Vega is 40 times brighter than the Sun. At this point, we will anticipate another question related to this answer: "How do we know that Vega is intrinsically brighter than Sirius?" To determine a star's true brightness, we first have to measure its distance. Vega and Sirius are so close we can determine their distances through the parallax method. For more information about this method, please refer to: http://usm.maine.edu/planet/could-you-please-explain-parallax-again-how-does-it-work-when-youre-measuring-distances-stars If we know the star's distance, we can then observe how bright it appears to us. We refer to this apparent brightness as its "apparent magnitude." By knowing a star's distance and its apparent magnitude, we can determine its absolute magnitude. "Magnitude" is the scale astronomers use to specify a celestial object's brightness. It is an inverse scale: the lower the number the brighter the object. So, for instance, a star of magnitude 1 will be brighter than a star of magnitude 2, which is brighter than a star of magnitude 3. The actual factor is 2.5, so a star of magnitude 1 will be 2.5 times brighter than a star of magnitude 2. Apparent magnitude measures how bright a star appears. Absolute magnitude measures how bright a star would appear at a distance of 10 parsecs, or about 32.6 light years. Absolute magnitude directly relates to a star's actual brightness, or luminosity. If we can know the star's distance and its apparent brightness, we can work out the star's intrinsic brightness. As an analogy, imagine you have two lights of varying brightness positioned at different distances across a football field. You can measure the distance to the lights by counting the yard markers separating you from them. You can clearly see how bright the lights appear. From these two pieces of information one can ascertain the relative brightnesses of the lights, thereby determining which is the brighter of the two. If you want more math, please refer to a math zone page pertaining to the distance modulus. http://usm.maine.edu/planet/mz-3-using-distance-modulus-calculate-suns-brightness-various-distances We hope these answers were helpful. *We know that to some people these mathematical footnotes seem as warm and friendly as 130-year old school marms armed with a gallon of stomach-dissolving Castor oil. We'll therefore try to proceed gently. For stars which are at least twice as massive as the Sun, but less than twenty times as massive, a star's luminosity is proportional to the mass raised to the power of 3.5. So, for instance, a star that is four times as massive as the sun will be 11 times more luminous than a star that is twice as massive. Even though Vega is 2.14 times as massive as the Sun, whereas Sirius is 2.0 times more massive, that difference will make Vega 1.6 times more luminous than Sirius. -Written by Edward Gleason Manager, Southworth Planetarium