July 24, 2014

Astro-Challenge: Hunting Przybylski’s Star.

Looking south from 28 degrees north an hour after sunset in mid-June.

(Click image for full view; created by the Author in Starry Night).

This week, we turn the astronomical spotlight on a seemingly ordinary star with an extraordinary tale. Often in visual astronomy, simply understanding the amazing facts behind a speck of light at the eyepiece can give it a special significance. Take last week’s transit of Venus, for example. Yes, it was merely a black dot taking almost 7 hours to cross the Sun. Not as splashy or dramatic as a total solar eclipse, sure, but the rarity and the historical significance was part of the fun. Same goes for some previous targets we’ve tracked down in the column, such as white dwarfs and quasars. Sure, you can read the astrophysical journals, but we always like to see this stuff in person. This week, we’d like to introduce you to a star that stubbornly defies astrophysics; Przybylski’s Star.

Located in the constellation Centaurus, this 8th magnitude star just scrapes above the southern horizon about 14 degrees in elevation for northern hemisphere observers around 30° north on June nights about an hour after local sunset. Fans of this site will remember our hunt for another famous Centaurus object, the massive cluster Omega Centauri. Both are visible in late spring/early summer months from mid-to-low northern latitudes, and if you’re around Miami area, you might just be able to nab the northern-most star Gamma Crucis of the Southern Cross asterism in the late spring time as well.

But it’s the non-descript Przybylski’s Star that we’d like to turn your attention (and perhaps your telescope) towards this week. This star is an enigma in terms of its chemistry. It was Antoni Przybylski that first noted the anomalous nature of this star in 1961. About 4 times the mass of our Sun, this star has abnormally low abundances of iron & nickel in its spectra and instead displays high amounts of exotic metals such as uranium, thorium, yttrium and a whole slew of other lanthanides. The fusion process runs out at iron, and all of these heavy elements had to have been forged  in supernovae sometime in the distant past… just what’s going on with Przybylski’s Star? This odd mixture puts the star in a rare category known as an Ap or Bp star, or stars with a high abundance of exotic metals, of which only 35 are known. These stars are well off the main sequence of the Hertzsprung–Russell diagram, and would fall “off the chart” past Delta-Scuti type variables and technetium laden stars. Pryzbylski’s star also displays a mind bogglingly fast pulsation rate of 12 minutes, discovered in 1978.

The star also displays another bizarre trait that may give some clues as to its troubled past; its moving at a slightly higher than usual velocity of about 24 kilometers per second with respect to neighboring stars. To give you some perspective, our own Sun is moving about 20 kilometers per second roughly towards Vega. Could Przybylski’s star have bared witness to several supernovae early in its career, salting it with heavy elements and ejecting it at high velocity? Whatever the case, Przybylski’s star is an astrophysical oddity worth hunting down in the Centaurus region. Its located 6.5° degrees northwest of Delta Centauri and its coordinates are;

Right Ascension: 11h 37’ 37”

Declination: -46° 42’ 35”

Przybylski’s star is about 370 light years distant.

Oh, and the second great mystery (and usually the first question we receive!) is “how the heck to you pronounce Przybylski?” Well, ye ole Internet search turns up a Polish pronunciation of Sh-eh-bel-skEE, though we always welcome a correction on our rusty Polish!

20.03.10: Spying a Black Hole Welterweight.

An Artist's conception of NGC 5408 X-1. (Credit: NASA).

An Artist's conception of NGC 5408 X-1. (Credit: NASA).

  

   Astronomers now have observational evidence for a missing class of black hole. Stellar mass black holes, those up to about 10 solar masses, are well known as the remnants of supernovae. Likewise for supermassive black holes of 10,000 solar masses or greater known to reside in the hearts of galaxies like our own. The “missing link” in astrophysics has been intermediate mass black holes, or those between 100 and 10,000 solar masses. Now, scientists at the Goddard Space Flight Center in Greenbelt Maryland have used the XMM-Newton and Swift X-ray satellites to pinpoint a likely candidate; NGC 5408 X-1, a black hole with about 1,000 to 9,000 solar masses in a galaxy about 15.8 million light years away in the constellation Centaurus. This would include an event horizon about 3,800 to 34,000 miles across. An X-ray flux occurs once every 115.5 days, strongly suggesting that NGC 5408 X-1 has a stellar companion accreting donor material. This star would be 3-5 times the Sun’s mass.   “Astronomers have been studying NGC 5408 X-1… because it’s one of the best candidates for an intermediate mass black hole.” States Philip Kaaret of the University of Iowa. The contributing companion also gives astronomers the unique opportunity to probe the near-space environment as well as study this intermediate class of enigmatic objects.