THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet <http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A> 70 Falmouth Street Portland, Maine 04103 43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970 Julian Date: 2459039.16 2019-2020: CLXXIII THE DAILY ASTRONOMER Wednesday, July 8, 2020 Remote Planetarium 61: A Backyard Review Well, first, our apologies to those who read these Remote Planetarium lessons only for the mythology. Today, we've decided to remain in the backyard where we shall admire the sky and review some of what we've already learned during the last twelve weeks. As we're firmly planted on Terra Firma, we'll have no access to the mythological realm hanging above us in the wide black yonder. We'll return to the deep sky tomorrow and will likely encounter something mythological. Welcome, by the way. You are standing on a vast tract of land attached to an 18th century farmhouse now silhouetted against the fading twilight. Our purpose today is to merely examine the sky and discuss what we now know about it. Consider this day something of a respite from the travels that have only just begun. We wanted to return just to help us understand how our perspective on the night sky has changed by virtue of learning astronomy. [image: Screen-Shot-2018-06-04-at-1.55.18-PM-e1528134961914.png] Let's regard the Summer Triangle. One can see it high in the eastern sky this time of year. The stars Deneb, Vega and Altair mark the triangle's three corners. As these three stars are each part of a different constellation, the Summer Triangle isn't a constellation, itself, but forms a star pattern. Deneb is part of Cygnus the Swan, Vega marks the northern tip of Lyra the Harp and Altair represents the eye of Aquila the Eagle. Without knowing much astronomy, one can still appreciate the beauty of these remote spheres. However, with the trained eye, one can discern much more about them. For instance, Vega's declination is almost +39 degrees, meaning that it is located almost 39 degrees north of the celestial equator. By knowing only this value, we can determine: - Where Vega passes directly overhead (at the zenith): *39 degrees N* - Vega's region of visibility: *Everywhere north of 51 degrees S* - Where Vega is circumpolar (always visible): *Everywhere north of 51 degrees N* What else can we determine about Vega? - 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. 1. 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: [image: toX0hNyGBcjCaTEzxL.0Fg_b.png] 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. To subscribe or unsubscribe from the Daily Astronomer: https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A= <https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=1>