Sky Shorts: Chart could help pick out stellar stars

Our April night sky showcases some of the most interesting stars that we see all year. This month features 11 stars that are first magnitude or brighter. In order of brightness are Sirius, Arcturus, Vega, Capella, Rigel Procyon, Betelgeuse, Aldebaran, Spica, Pollux and Regulus. The word magnitude refers to a celestial object’s apparent brightness or apparent visual magnitude as seen from Earth. For our human eyes, the brightest stars are 1st magnitude, and the dimmest stars are 6th magnitude. Around 2,000 years ago, ancient astronomers Hipparchus and Ptolemy devised the magnitude scale that we still use today, with some modification.

But since the distance between a star and Earth can affect the light we see; astronomers now use the term absolute magnitude. Absolute magnitude is the apparent magnitude if we image a star is 10 parsecs or 32.6 light years from Earth. This allows astronomers to compare stars on the same footing.

Luminosity also determines brightness. Luminosity is the amount of light a star radiates, which is determined by the size and surface temperature of a star.

Other characteristics of stars include color, surface temperature, size and mass. A star’s color indicates surface temperature. Hotter stars are blue in appearance while cooler stars are red. Stars in the mid-range are white or yellow like our Sun. Stars can also be a combination of colors such as red orange, like Betelgeuse, or blue, white, like Rigel. Surface temperature is measured using the Kelvin scale, proposed by William Thomson. Zero degrees on the Kelvin scale is equal to -276.15 degrees Celsius. The coolest stars are about 2,500 K, while the hottest are 50,000 K. Our Sun is around 5,000 K. Astronomers measure stars in terms of the size of our Sun, which represents 1 solar radius. So, a star the same size as our Sun would be 1 solar radii. Mass works in a similar way. One solar mass is equal to the mass of our Sun.

In the early 1900s, two astronomers, Ejnar Hertzsprung and Henry Norris Russell, working independently, created a diagram which combined all of the characteristics of stars, called the Hertzsprung-Russell Diagram. The HR diagram is one of the most important tools in the study of stellar evolution. It is a plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. Stellar classification is a system created by Annie Jump Cannon to categorize stars depending on their mass and temperature. The classification of stars has evolved into seven distinct classes known as O, B, A, F, G, K and M. Star’s classified in the “O” group are the most massive and hottest and the “M” group are the smallest and coolest.

On the H R Diagram, you can see that stars gather in groups. 90% of all stars are Main Sequence stars. Our Sun is a part of that group, along with our April stars, Regulus, Sirius, Vega, Castor and Spica which are hot, bright stars, while Procyon is like our Sun. Our April Giants include Capella, Arcturus, Pollux and Aldebaran, which is the coolest of the group.

Finally, the April Supergiant’s are Betelgeuse (cooler) and Rigel (hot). Betelgeuse is one of the largest stars and is approximately 950 times the size of our Sun. If we replaced our Sun with Betelgeuse, it would reach all the way to the orbit of Jupiter. Rigel is a very hot Supergiant and is thousands of degrees hotter than our Sun. Because Rigel lives such a fiery existence, it will only live for 10 million years, whereas our Sun will live for 10 billion years. Both Betelgeuse and Rigel will supernova at the end of their lives and both can be found in the constellation Orion, the Hunter.

So, on the next clear April night, grab a starmap or favorite night sky app and marvel at these fascinating stars!

NIGHT SKY FOR APRIL

Planets and the Moon: 

Mars continues to rule the evening sky through April. Mars is high in the western sky after sunset, passing through the constellation Taurus, the Bull, and then on April 25, passes less than a degree north of Messier 35, a beautiful star cluster. Venus and Mercury make an appearance late in the month in the west, northwest right after sunset. Binoculars will work best, and visibility will improve next month. In our predawn sky, Jupiter and Saturn are low in the southeast and will continue to climb higher each morning. At the beginning of April, Jupiter rises two hours before sunrise and three hours before at month’s end. Visibility for Uranus and Neptune will improve in the coming months. April’s Full Moon, on April 26, will be the second biggest and brightest for 2021, known as a Supermoon. The Crescent Moon will pass south of Saturn on April 6 and south of Jupiter on April 7. The Crescent Moon will then pass south of Mars on April 17. The Lyrid Meteor Shower peaks on April 22 but the Waning Gibbous Moon will interfere with the fainter shooting stars.

Constellations:

West: Look for the three stars in a line, which make up the belt of Orion. The bright red-orange star up and to the left of the belt is Betelgeuse. The bright blue-white star down and to the right of the belt is Rigel.  Draw a line to the right, from the belt, to a red, orange star, Aldebaran, which is the eye of Taurus, the Bull. Aldebaran is part of the V shape of stars, which is the face of Taurus. To the right of Taurus, you will find the small cluster of stars, the Pleiades or Seven Sisters. Making a counterclockwise loop from the Pleiades, the next bright star is Capella. Continuing up, the two stars you see are Gemini, the Twins.

North: The Big Dipper is high in the sky. Following the two stars at the end of the cup to the next bright star, Polaris, or the North Star. The constellation Cassiopeia is down and to the left of Polaris and resembles a sideways letter ‘M”.

Overhead to East: Head back to the cup of the Big Dipper. Locate the flat part of the cup. Look high overhead for the shape of a backwards question mark. This is the head of Leo, the Lion. Now, arc off the handle of the Big Dipper to the bright yellow, orange star Arcturus. Look right for the bright blue, white star, Spica.

Binocular Highlights: Facing west, you will see the small cluster of stars, the Pleiades or the Seven Sisters. The Pleiades is a beautiful open star cluster.  Head to Orion, the Hunter. Scan below the three stars of Orion’s belt. You will see fuzzy area with bright stars. This is the Orion Nebula, a hydrogen gas cloud where new stars are forming. For a challenge, scan between Leo and Gemini. There you will find the Beehive Star Cluster.

For further night sky details, maps and audio, visit my website www.starrytrails.com.

VISIT HOOVER PRICE PLANETARIUM  Visit www.mckinleymuseum.org, for limited show dates and times! Planetarium shows are free with Museum admission. Seating is limited and will be on a first come, first serve basis. The Planetarium is located inside the McKinley Presidential Library & Museum, 800 McKinley Monument Drive, N.W., in Canton, Ohio. For more information, please call the Museum at 330-455-7043.