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

              "A day without sunshine is, like, you know, night."

                                        -Steve Martin

 

 

THE DAILY ASTRONOMER

Monday, March 28, 2016

Stellar Pandora

 

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"Fantastic Physics"  begins tomorrow!

A Southworth Planetarium day course that combines hands-on activities and lectures to elucidate the fundamental principles of physics: the science of how the world works! 

Six Tuesday mornings  10:30 a.m. - noon

http://usm.maine.edu/planet/fantastic-physics

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A triple Pandora event today!

We reached into Pandora's Jar to find three questions related to the stars, the building blocks of our galaxy and Universe.     We still consider it a three-saint miracle that humans were ever able to understand stars in the first place, as they are so remote from us.  Recall that no human has strayed more than about a quarter of a million miles from Earth, while even the closest star, the Sun, is about 93 million miles away.     However, astronomy has enabled humans to understand the physics, life cycles and various properties of the stars despite being confined to a small world around an average star.

 

 

"I received an e-mail that said, 'Because it takes so long for their light to reach Earth, many of the stars we see at night are long gone.'   Is this statement true?"

-S. Kirk,  Richmond, VA

 

Most likely not.    It is true that stars are quite far away.  Also, even light emanating from the closest stars requires years to reach us.  (The light from Alpha Centauri, the closest star to our solar system, takes 4.2 years to travel to Earth.)   The most distant naked eye star, V762, a faint variable star in Cassiopeia, is about 16,300 light years away. However, the life spans of even the shortest-lived stars exceeds 10 million years.   These fleeting stars are the most luminous of the supergiants, as a star's life span is inversely proportional to its mass and luminosity.  The more massive and more luminous, the shorter the life cycle.      

 

Almost all of the stars we see with the unaided eye are intrinsically bright stars. Many appear faint because of their distances.    However, very few of them are at the end of their life cycles.  Two notable exceptions are Betelgeuse (Orion the Hunter) and Antares (Scorpious the Scorpion.)  These stars are highly massive red supergiants that will eventually explode as supernovae within the next million years.   Of course, they might have done so already, but still appear to us because the light from such explosions would require centuries to reach Earth. Betelgeuse and Antares are 620 light years and 550 light years away, respectively.     

It is possible that Betelgeuse and/or Antares have gone supernova and no longer exist.    The rest, however, are most likely still with us!

 

 

"Where is the nearest Sun-like star to the Sun?"

Surprisingly close!   First, we should explain that our Sun is a G2 V star.    The "G2" refers to its spectral type.   Astronomers classify stars according to their spectral types (or temperatures) which form a sequence  O B A F G K M,* with O stars being the hottest and M stars the coolest.    This sequence is futher calibrated numerically so, for instance, a G8 star is slightly different from a G2 star.   The Roman numeral refers to a star's luminosity class.   The "V" denotes 'dwarf' status.   Yes, though the Sun is immensely large by our standards, it is actually a dwarf star.   (A bright giant is designated with II; a 'normal' giant with III.  A luminous supergiant is assigned a Ia, to cite other examples.)    So, we could say that any G2 V is certainly "Sun like."  The closest G2 V star is Alpha Centauri A,  which, at distance of 4.2 light yeas,  is part of the closest star system to the Sun.   Our closest neighbor is like the Sun.

Astronomers tend to pay close attention to such Sun-like stars because many assume that they could be the parent stars of habitable planets.     Unfortunately, Alpha Centauri A is part of a ternary (three star) system and so far no exo-planets have been discovered orbiting Alpha Centauri's component stars.      However, the closest single "Sun-similar" (G8 V) star, Tau Ceti (distance: 12 light years) has five known exo-planets, one of which orbits within the star's "habitable zone," a region where temperatures might be  conducive to life's development.     Although no evidence of life has yet been found in the Tau Ceti system, astronomers are searching for "bio signatures," indications of chemicals, such as methane and molecular oxygen, which are associated with metabolic activity.    

 

 

 

"I know that astronomers have found a lot of planets around single stars.  Have planets been found around triple or quadruple stars?"

         John F.,  Portsmouth, NH

 

Yes!  Planets have been found around triple and quadruple star systems.    In fact, astronomers found the first planet in a triple star systems in 2005! This planet was found within the star system HD 188753.   The planet revolves around the primary.   The other two stars are locked in a tight orbit away from the primary.     The triple star system GJ 667 possesses a planet that is also locked in orbit around a single component with the system.   

 

We also know of two quadruple star systems that have planets.  The first was found in 2013 within the quadruple system KIC  4862625.    Astronomers found another in the quadruple star system 30 Ari, which is about 136 light years away within the constellation Aries.     In both systems, the planet revolves around only one component star.

 It is unlikely that astronomers will find a planet that orbits all the stars within a multiple star system, as such orbits would be quite unstable.

 

*You can remember this sequence with the mnemonic  "Oh, Be A Fine Girl, Kiss Me!"   

 

**In 2012, astronomers with the HARPS Observatory announced the discovery of the exo-planet Alpha Centauri Bb, a world purported to be in a tight orbit around Alpha Centauri B.      Further analysis has revealed that no such planet exists, so, for now, we know of no planet within the Alphe Centauri system.    We are not suggesting, however, that there are no planets within Alpha Centauri.    Astronomers have found stars in multiple star systems.  (Refer to the third question in today's article.)