NASA Space Place Column, April 2015

Is the Most Massive Star Still Alive?

By Ethan Siegel

The brilliant specks of light twinkling in the night sky, with more and more visible under darker skies and with larger telescope apertures, each have their own story to tell. In general, a star's color correlates very well with its mass and its total lifetime, with the bluest stars representing the hottest, most massive and shortest-lived stars in the universe. Even though they contain the most fuel overall, their cores achieve incredibly high temperatures, meaning they burn through their fuel the fastest, in only a few million years instead of roughly ten billion like our sun.

Because of this, it's only the youngest of all star clusters that contain the hottest, bluest stars, and so if we want to find the most massive stars in the universe, we have to look to the largest regions of space that are actively forming them right now. In our local group of galaxies, that region doesn't belong to the giants, the Milky Way or Andromeda, but to the Large Magellanic Cloud (LMC), a small, satellite galaxy (and fourth-largest in the local group) located 170,000 light years distant.

Despite containing only one percent of the mass of our galaxy, the LMC contains the Tarantula Nebula (30 Doradus), a star-forming nebula approximately 1,000 light years in size, or roughly seven percent of the galaxy itself. You'll have to be south of the Tropic of Cancer to observe it, but if you can locate it, its center contains the super star cluster NGC 2070, holding more than 500,000 unique stars, including many hundreds of spectacular, bright blue ones. With a maximum age of two million years, the stars in this cluster are some of the youngest and most massive ever found.

At the center of NGC 2070 is a very compact concentration of stars known as R136, which is responsible for most of the light illuminating the entire Tarantula Nebula. Consisting of no less than 72 O-class and Wolf-Rayet stars within just 20 arc seconds of one another, the most massive is R136a1, with 260 times the sun's mass and a luminosity that outshines us by a factor of seven million. Since the light has to travel 170,000 light years to reach us, it's quite possible that this star has already died in a spectacular supernova, and might not even exist any longer! The next time you get a good glimpse of the southern skies, look for the most massive star in the universe, and ponder that it might not even still be alive.

Images credit: ESO/IDA/Danish 1.5 m/R. Gendler, C. C. Thöne, C. Féron, and J.-E. Ovaldsen (L), of the giant star-forming Tarantula Nebula in the Large Magellanic Cloud; NASA, ESA, and E. Sabbi (ESA/STScI), with acknowledgment to R. O'Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee (R), of the central merging star cluster NGC 2070, containing the enormous R136a1 at the center.

NASA Space Place Column, March 2015

The Cold Never Bothered Me Anyway

By Ethan Siegel

For those of us in the northern hemisphere, winter brings long, cold nights, which are often excellent for sky watchers (so long as there's a way to keep warm!) But there's often an added bonus that comes along when conditions are just right: the polar lights, or the Aurora Borealis around the North Pole. Here on our world, a brilliant green light often appears for observers at high northern latitudes, with occasional, dimmer reds and even blues lighting up a clear night.

We had always assumed that there was some connection between particles emitted from the Sun and the aurorae, as particularly intense displays were observed around three days after a solar storm occurred in the direction of Earth. Presumably, particles originating from the Sun—ionized electrons and atomic nuclei like protons and alpha particles—make up the vast majority of the solar wind and get funneled by the Earth's magnetic field into a circle around its magnetic poles. They're energetic enough to knock electrons off atoms and molecules at various layers in the upper atmosphere—particles like molecular nitrogen, oxygen and atomic hydrogen. And when the electrons fall back either onto the atoms or to lower energy levels, they emit light of varying but particular wavelengths—oxygen producing the most common green signature, with less common states of oxygen and hydrogen producing red and the occasional blue from nitrogen.

But it wasn't until the 2000s that this picture was directly confirmed! NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite (which ceased operations in December 2005) was able to find out how the magnetosphere responded to solar wind changes, how the plasmas were energized, transported and (in some cases) lost, and many more properties of our magnetosphere. Planets without significant magnetic fields such as Venus and Mars have much smaller, weaker aurorae than we do, and gas giant planets like Saturn have aurorae that primarily shine in the ultraviolet rather than the visible. Nevertheless, the aurorae are a spectacular sight in the evening, particularly for observers in Alaska, Canada and the Scandinavian countries. But when a solar storm comes our way, keep your eyes towards the north at night; the views will be well worth braving the cold!

Image: Auroral overlays from the IMAGE spacecraft. Image credit: NASA Earth Observatory (Goddard Space Flight Center) / Blue Marble team.

Friday March 13 GAAC Meeting: Cassini and the Moons of Saturn

This month GAAC is fortunate to host Robert Naeye, former Editor in Chief of Sky & Telescope, the world’s most respected and influential popular astronomy magazine. Robert will be speaking to us Friday, March 13, on the Cassini mission to Saturn and its moons.

In July 2004, NASA’s Cassini spacecraft fired its braking rocket and entered orbit around Saturn. Since then, Cassini has orbited the Ring Planet hundreds of times, and returned hundreds of thousands of images, many of which we will see on the 13th, along with a flood of data about Saturn’s magnetic field, particle environment, and ring composition. This enormous dataset has revolutionized science’s understanding of the Saturnian system.

Besides studying Saturn and its rings, Cassini has unveiled its mysterious moons, showing the planet and moons to be a mini-solar system unto itself. In 2005, Cassini deployed the European-built Huygens probe, which parachuted and landed on the surface of Saturn’s largest moon, Titan, arguably the most Earth-like world in the solar system other than Earth itself. Cassini and Huygens have revealed Titan to be a world of complex meteorology and geology, with lakes and rivers fed by methane rainfall.

Perhaps most exciting of all, Cassini has also found jets of water-ice particles laced with organics shooting away from the moon Enceladus, making this small world a potential abode for life. And Cassini images of Iapetus have helped explain how this bizarre moon got its yin-yang appearance, with one side darker than coal and the other as bright as freshly fallen snow.

Robert Naeye earned a master’s degree in science journalism from Boston University in 1992, and later worked on the editorial staffs of Discover and Astronomy magazine. He served as Editor in Chief of Mercury magazine (published by the Astronomical Society of the Pacific) from 2000 to 2003. He worked as a Senior Editor at Sky & Telescope from 2003 to 2007, before moving to NASA’s Goddard Space Flight Center to work as a Senior Science Writer for the Astrophysics Science Division. He returned to Sky & Telescope in June 2008 to serve as Editor in Chief.

Robert is the author of two books: Through the Eyes of Hubble: The Birth, Life, and Violent Death of Stars (Kalmbach, 1997) and Signals from Space: The Chandra X-ray Observatory (Turnstone, 2000). He has contributed to two other books, and has won several awards for his writing and outreach activities.

Our Spring 2015 Series of Public Astronomy Presentations

GAAC February 13 Program Note -- Space Telescopes!

We have an absolute monster of a program for our February GAAC meeting to begin the new GAAC year.

Many of those wonderful pictures of objects in space -- galaxies spinning, black holes feeding, stars being born and dying, planets moving around other stars, and so many others, are taken by a fleet of space telescopes.

We've all heard of the Hubble, but we actually have different space telescopes to observe different parts of the electromagnetic spectrum -- light -- from Gamma rays through our rainbow to the Infrared.

Why are the different space telescopes used and what do we learn from them? What do the different pictures look like? What is the future of space telescopes? We have the answers for you.

On Friday, February 13, North Shore Amateur Astronomy Club president and long-time GAACster Kevin Hocker will show us examples of different space telescopes that do different jobs, and, of course, we'll see some of their amazing discoveries.

The space telescopes we will hear about include:

  • Swift Gamma Ray Burst Telescope
  • Chandra X-Ray Observatory
  • Hubble Space Telescope
  • Kepler - the telescope in the hunt for Earth like planets
  • Spitzer Infrared observatory
  • And of course, the Future of space telescopes - James Webb Space Telescope

Kevin will show us an overview of each telescope, its features, and of course a striking and colorful assortment of their discoveries. Don't miss this one! You'll be telling your friends all about this for weeks!

Sky Object of the Month – February 2015

NGC 1501 – Planetary Nebula in Camelopardalis
by Glenn Chaple

While Go-to technology has gained popularity with backyard astronomers who like to key their telescopes on a sky object with the push of a button, I prefer the no-frills star-hop mode of cosmic travel. Star-hopping lets me see enjoy celestial scenery I’d miss by traveling Go-to. I’ll demonstrate my point with a star-hop to the planetary nebula NGC 1501 in Camelopardalis.

Camelopardalis isn’t very kind to star-hoppers. This sprawling north circumpolar constellation contains just four stars brighter than 5th magnitude. A star-hop to any sky destination in Camelopardalis usually begins with a bright star in an adjacent constellation. To find NGC 1501, we begin at gamma () Persei and trace a 12o path between a pair of 4th magnitude stars to Kemble’s Cascade (refer to the finder charts on the right).Kemble’s Cascade is a stunning 2 ½ o chain comprised of some 20 magnitude 7 to 9 stars.

At its southwest end is the pretty open cluster NGC 1502, punctuated at the center with the eye-pleasing 7th magnitude twins that make up the double star Struve 485. A 1 ½ o push south of NGC 1502 brings us to NGC 1501. Think of it – if we’d traveled to NGC 1501 via Go-to technology, we’d have missed three delightful celestial showpieces!

NGC 1501 is a magnitude 11.5 planetary nebula located about 5000 light-years away. Its slightly oval disk, just under an arc-minute across, can be glimpsed (barely) in a 3-inch scope, but twice that aperture will be needed for a definite sighting.  With a 12-inch scope and dark-sky conditions, you should be able to make out the nebula’s bluish hue and magnitude 14.5 central star.

NASA Space Place Column, January 2015

Minor mergers have massive consequences for black holes
By Dr. Ethan Siegel

When you think of our sun, the nearest star to our world, you think of an isolated entity, with more than four light years separating it from its next nearest neighbor. But it wasn't always so: billions of years ago, when our sun was first created, it very likely formed in concert with thousands of other stars, when a giant molecular cloud containing perhaps a million times the mass of our solar system collapsed. While the vast majority of stars that the universe forms—some ninety-five percent—are the mass of our sun or smaller, a rare but significant fraction are ultra-massive, containing tens or even hundreds of times the mass our star contains. When these stars run out of fuel in their cores, they explode in a fantastic Type II supernova, where the star's core collapses. In the most massive cases, this forms a black hole.

Over time, many generations of stars—and hence, many black holes—form, with the majority eventually migrating towards the centers of their host galaxies and merging together. Our own galaxy, the Milky Way, houses a supermassive black hole that weighs in at about four million solar masses, while our big sister, Andromeda, has one nearly twenty times as massive. But even relatively isolated galaxies didn't simply form from the monolithic collapse of an isolated clump of matter, but by hierarchical mergers of smaller galaxies over tremendous timescales. If galaxies with large amounts of stars all have black holes at their centers, then we should be able to see some fraction of Milky Way-sized galaxies with not just one, but multiple supermassive black holes at their center!

It was only in the early 2000s that NASA's Chandra X-ray Observatory was able to find the first binary supermassive black hole in a galaxy, and that was in an ultra-luminous galaxy with a double core. Many other examples were discovered since, but for a decade they were all in ultra-massive, active galaxies. That all changed in 2011, with the discovery of two active, massive black holes at the center of the regular spiral galaxy NGC 3393, a galaxy that must have undergone only minor mergers no less than a billion years ago, where the black hole pair is separated by only 490 light years! It's only in the cores of active, X-ray emitting galaxies that we can detect binary black holes like this. Examples like NGC 3393 and IC 4970 are not only confirming our picture of galaxy growth and formation, but are teaching us that supermassive relics from ancient, minor mergers might persist as standalone entities for longer than we ever thought!

Check out some cool images and artist reconstructions of black holes from Chandra:

Kids can learn all about Black Holes from this cool animation at NASA’s Space Place:

Images credit: NGC 3393 in the optical (L) by M. Malkan (UCLA), HST, NASA (L); NGC 3393 in the X-ray and optical (R), composite by NASA / CXC / SAO / G. Fabbiano et al. (X-ray) and NASA/STScI (optical).

Sky Object of the Month – January 2015

Struve 817 (STF 817, 817) – Double Star in Orion
by Glenn Chaple

I’m a big fan of “off-the-beaten-path” sky objects. One of my favorites is the little-known double star Struve 817 - the 817th double star catalogued by the German-born Russian astronomer F. G. W. Struve during a survey conducted between 1824 and 1827. I wrote about this little gem in my first “Object of the Month” column 5 years ago. It’s time for a return visit!

According to a measure made in 2010 and posted in the Washington Double Star Catalog (available online at, Struve 817 consists of near-twin magnitude 8.68 and 8.93 stars separated by 18.7 arc-seconds in a position angle of 73o. The separation and P.A. differ little from what Struve himself measured around the time of discovery. Astronomers describe stellar partnerships that show little orbital motion as being “relatively fixed.” If the component stars of Struve 817 form a true binary pair, their orbital period must encompass many centuries.

What gives this relatively obscure double star a special allure is its location just 20 arc-minutes south of the red supergiant Betelgeuse. To find Struve 817, simply aim your telescope at Betelgeuse. A medium power eyepiece (75 to 100X works well) should capture this delicate pair shining just outside the dazzling rays of ruddy Betelgeuse. It’s a startling sight. The Washington Catalog lists the spectra of Struve 817’s components as A5 and K. Can you make out a color contrast between the two?

Some years ago, I wrote a four-part seasonal series for Deep Sky Magazine in which I introduced my favorite 100 double stars. Included with such celebrated pairs as Mizar, Albireo, and the “Double-double” epsilon Lyrae was Struve 817. On the next crisp winter night when Orion beckons you to visit his magnificent Nebula, take a minute to travel a road less taken and try for this delightful double star.

Image credits: (courtesy IAU and Sky and Telescope) Betelgeuse and Struve 817. 3-inch f/10 reflector at 60X; ½ degree field.