Celestial Greetings from the Milky Way Cowboys at Bare Dark Sky Observatory (BDSO)!
As long as we’re still in social lock down, we decided to send a short supplemental astronomy lesson for those who want to enhance their night sky knowledge and experience. While we know that you can research this information on the Internet, and we encourage it, we just want to stay connected to the thousands of people who have visited or contacted us. We miss hosting the observatory events like crazy and hope to meet up with many of you again soon!
When you see a planet in the sky it can be difficult to distinguish it from a star. But there is one subtle difference you may be able to notice the next time you look. Remember the childhood song mom used to sing to you at bedtime … “Twinkle, Twinkle Little Star?” It’s true! Unlike stars, planets don’t twinkle. Stars are incredible gigantic thermal nuclear reactors that generate their own light. But they are so distant they appear as pinpoints of light after traveling for hundreds and thousands (maybe millions) of years to reach our eyes. Because all of a star’s light is coming from a single point, it is easily diffracted by Earth’s atmospheric interference.
Conversely, we see planets and moons because they are right here in our solar system, close enough to reflect the Sun’s light. Because the light reflected is bigger than the point of light from a star, the planet will not appear to twinkle. The proportion of light that we see from a surface reflection is called “albedo.” When we look at Venus, it shines brightly because about 75% of the Sun’s light is reflected by its highly reflective clouds of sulfuric acid. Our full Moon only has about 12% albedo. That’s because the dark basaltic plains from ancient volcanic eruptions absorb the light and reflect little. Imagine how bright the Moon would be if it had 70 or 80% albedo! Earth’s 36% albedo varies because of snow and clouds. But generally, about two-thirds of the Sun’s light is absorbed by our water and land masses. One of Saturn’s 82 moons, Enceladus, is covered in a layer of fresh clean ice and has an albedo of over 90%, the highest in our solar system.
In our Star Gazing Guide, we referred to the “magnitude” of stars and planets. In very simple and not-too-technical terms, astronomers will describe the apparent magnitude of celestial objects as measurement of its brightness or luminosity as it appears in our night sky from Earth. The scale is logarithmic and defined such that each number of magnitude changes the brightness by a factor of about 2.5. An object that is magnitude +1 would be 2.5 times brighter than magnitude +2. An object of magnitude +2 would be 2.5 times brighter than one +3, and so on. A +1 magnitude star is 100 times brighter than a magnitude +6 and 10,000 times brighter than magnitude +10. The dimmest stars that can be seen without an optical aid, and with 20/20 vision and very good sky conditions, is around +6 magnitude. In short, the lesser the number, the brighter the object. Remembering this basic concept will help you determine what you can see when stargazing.
Examples of objects that we can easily see: Our Sun (-26), full Moon (-13), Venus (varies -3 to -4.6), Jupiter (varies -2 to -3), Saturn (varies +1 to -.5), Sirius in Canis Major (-1.46), Arcturus in Bootes (-.04), Vega in Lyra (+.03), Capella in Auriga (+.08), Rigel in Orion (+.12), Betelgeuse in Orion (+.42), and Polaris in Ursa Minor (+2). There are 22 stars in the mag +1 and under range.
A great example of how magnitude can vary is Betelgeuse, the red giant at Orion’s shoulder. It had dimmed dramatically (from +.42 to +1.6) over recent months and some have speculated that it might be a sign of the expected eventual supernova … the end of a star’s life when it collapses in on itself and then explodes dramatically. If that actually happened, we would probably see it both day and night and it would be much brighter than our half Moon. Wouldn’t that be a sight to behold!
Another wonderful example of how magnitude can vary is when we observe Mars. Currently, Mars can be seen in the early morning sky at about magnitude +.6. By October Mars will brighten dramatically to -3 or -4 in our night sky. The reason is because Mars is moving closer to Earth and we will be positioned in between Mars and the Sun (conjunction). Instead of being 140 million miles away from Earth, Mars will only be 38 million miles! If Mars’s wind storms aren’t severe, we should have an opportunity to see a stunning polar cap through our observatory telescopes.
The total number of naked eye visible magnitude +6 or less stars that can be observed in the night sky is 9,096. Since you can only see half of the celestial sphere at one time, we cut that number in half to approximately 4,500. And since we have a limited view of the earth from side to side, cut that down to under 3,000 in the mountains of North Carolina. If you live in the city lights of Charlotte, Washington D.C., or Atlanta, the number of visible stars drops dramatically to just a few hundred. We’re fairly certain this is why so many of you have visited the BDSO. With binoculars, which can help us see up to magnitude +10, we are able to see about 300,000 stars at our observatory. In our 14” mirror Meade telescope, we can see up to 25 million stars (+14 mag) and up to 700 million (+18 or +19 mag) in our 34” mirror “Sam” scope. Imagine how many stars the Hubble Telescope sees with an eight foot mirror at +30 magnitude!
Our guests often ask why stars (besides our Sun) do not appear larger in our telescopes. Answer: Stars are so far away that only the largest telescopes here on Earth are going to show them bigger. What our telescopes and binoculars enable us to do is see many, many more stars. Looking at open and closed star clusters, nebulae, galaxies, and of course, our sister planets and their moons, can take us on incredible night sky adventures
where no one has gone before.
And why is that? Why have we not yet traveled to the nearest star? Well, the nearest star system, Proxima and Alpha Centauri (in the constellation Centaurus seen in the southern hemisphere), is 4.22 light years away. Remember that light travels at 186,282 miles per second, or about 5.9 trillion miles per year (a light year). For us to make that star trek of about 25 trillion miles, would take us well over 100,000 years at the current speed of our best space craft. Investigating our Moon and Mars first hand is still our best option for now.
Hopefully, this session has been a source of enlightenment to you and will inspire you to spend more time under the night sky with family and friends.
P.S. Don’t forget to check out the Lyrid Meteor Shower after midnight on April 20 & 21!
