Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration.
Residing about 6,500 light years away in the southern constellation Puppis is this magnificent and seemingly mysterious object, shrouded in a cocoon of stellar material. This image comes to us from the legendary Hubble Space Telescope (HST) and it’s a Cepheid Variable Star called “RS Puppis.”
Don’t worry too much about the “RS” and “Cepheid” classifications as there is a vast list of various types of variable stars (link below). Just know that variable stars are stars that over a regular given period of time brighten and dim slightly. Cepheid’s in short, brighten greatly over a relatively long period. RS Puppis has a brightness of 15,000 times that of our host star, is roughly 20 times its mass and about 200 times larger. Its variability cycle or pulsation takes about 41.4 days or 6 weeks, resulting in an apparent magnitude range of a dim 7.6 to a naked eye visible at 6.5.
So what is a variable star? I’ll give you the “Cliff Notes” version. When a star begins to run low on hydrogen in its core, its millions or billions of years as a stable star begins to deteriorate. This instability causes the stars to swell and shrink over a given period of time. As they swell they brighten and when they shrink back down they get dimmer. When they shrink back down they also leave behind a massive shell of material that becomes the shroud or cocoon that HD 68860 here is surrounded by.
Because of their brightness and regularity Cepheid variable stars are what’s known as “standard candles.” These standard candles allow us to make high precision, distance measurements throughout the galaxy and universe.
Finally, in this image exists a fairly common variable star phenomenon known as “Light Echo.” Think first of what a standard sound wave echo is. If you yell or make an audible noise in a place with even the most rudimentary acoustics and in a moment’s time you hear your voice or that particular noise return to you after it has traveled out, bounced off something or multiple somethings and returned to you. A light echo is pretty much the same thing just with light instead of sounds. That sounds pretty unrealistic as light travels fast (7 times around the Earth per second) but the distances here are large enough so that even at 186,282 miles per second we can still see it in motion. The material in this star’s environment enables this effect to be shown with stunning clarity. As the star expands and brightens, we see some of the light after it is reflected from progressively more distant shells of dust and gas surrounding the star, creating the illusion of gas moving outwards. Want to see this in amazing detail? Then watch the Hubblecast 71 video above!
Name: HD 68860, SAO 198944, RS Puppis
What is it?: Cepheid Variable Star with a period of about 41.4 days
How big is it?: Roughly 10 stellar masses & 200 X larger than the Sun
How far away is it?: 6,500 light years or 2000 parsecs
Apparent magnitude?: 6.5 to 7.6
Where is it (general)?: Southern constellation Puppis
Where is it (exact RA/Dec J2000)?: R.A. 08h 13m 04s / DEC -34° 34′ 42.7”
NASA Hubble News Center: http://hubblesite.org/newscenter/archive/releases/2013/51/image/a/
ESA Hubble Space Telescope: http://www.spacetelescope.org/images/heic1323a/
ESA Hubble Space Telescope Information: http://www.spacetelescope.org/news/heic1323/
AAVSO Variable Star Type Designations: http://www.aavso.org/vsx/help/VariableStarTypeDesignationsInVSX.pdf
AAVSO RS Puppis Page: http://www.aavso.org/vsots_rspup