February 21, 2019

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!

21.04.10-The Puzzle of Blue Stragglers.

Astronomers may have recently solved a half a century long mystery of stellar evolution. Since the 1950’s a type of star known as  a blue straggler has stubbornly refused to fit the Hertzsprung-Russell diagram mold. These older stars should be approaching seniority, but instead burn brightly and spin energetically as if they had somehow gained mass. Most exist in globular or open clusters, and were first identified in the M3 globular cluster. The most well studied example of this stellar sub-class exist in NGC 188, a star cluster about 6,000 light years distant where 21 have been identified. Now, astronomers Robert Mathieu and Aaron Geller of the University of Wisconsin Madison have gained insight into the formation of these elusive beasties and come up with three leading hypotheses;

  1. Matter is accreted from an aging red giant star onto a main sequence companion similar to the process seen in a type IA supernova, but not as massive, causing the star to re-energize;
  2. Two lower mass stars collide, an improbable but not impossible scenario in a densely packed globular cluster;
  3. A third stellar companion perturbs the orbit of a tightly knit binary pair, causing them to merge.

These possibilities were advanced after Mathieu and Geller used observing time on the 3.5-meter WIYN telescope on Kitt Peak spanning the past decade. Studies involved NGC 188, the original “blue straggler” cluster. “These aren’t just normal stars that are straggling behind in their evolution,” stated Mathieu.” There is something unusual going on with their companions.” Computer models would suggest that door number #3 is the most likely candidate; the most logical proof that astronomers would like to have in hand would be to catch a merger in progress.  Interestingly, two known blue stragglers with white dwarf companions lie in the field of the Kepler space telescope, a plus for the accretionary camp. Will we soon have definitive evidence for the origins of these bizarre stars? Or is it perhaps a hybrid of the three models? Stay tuned…