ASTRONOMY UPDATES

ASTRONOMY UPDATE 2005

James B. Kaler

Department of Astronomy, University of Illinois

First published in the Proceedings of the 41st Annual GLPA Conference, Grand Rapids, MI, October 19-22, 2005. Reprinted by permission.

Abstract

Though Mars is always in the news, it was nearly eclipsed this year by a satellite, Saturn's Titan, whose surface was finally revealed, and by a new "planet" larger than Pluto that inhabits the outer extension of the increasingly populous Kuiper Belt. Much farther away we studied what appears to be kuiper belts or asteroid zones surrounding other stars, and more stars with orbiting planets, the low- mass record for which is a mere nine Earth-masses. From a greater distance astronomers noted a powerful blast from a "soft gamma ray repeater," created a new map of the Galaxy's spiral arms, saw bunches of dwarf galaxies around us, and watched the collisions of whole clusters of galaxies.

News from Earth

We begin with ourselves and our ability to observe, which is topped by Hubble, the fate of the telescope still "up in the air" as NASA decides what to do. Crashing it back down would be a tragedy for world astronomy, while fixing it is clearly dangerous, given the current technical and engineering problems of the Shuttle.

"Down in the air" is Yerkes Observatory, which is in a similar fix. Its historic instrument, the largest refractor on Earth, is in trouble, as the University of Chicago wants to sell the property to a developer, though it looks now as if the Observatory will be used for educational purposes.

Better news is the startup of LOFAR (the Low Frequency Array), which will eventually consist of 15,000 small antennas that will give the best resolution ever for as low a radio frequency as 10 Megahertz. The first data span the Milky Way from the Crab Nebula to Cassiopeia A. Gravity Probe B also continues along, though its eventual test of frame dragging (in which a rotating body's gravity drags spacetime along with it) was beaten out by a positive detection by the LAGEOS satellite. The very concept of frame dragging owes itself to Einstein's relativity, allowing us to celebrate his "wonder year" of 1905, in which he published papers on special relativity, Brownian motion, quantization of radiation, and other topics. He then began his long road to the discovery of Grand Rapids...oh, General Relativity and Gravitational Radiation...hard to tell the GRs apart...

Earth rotation has a far more obvious trait: it is erratically slowing down (over the long haul) as a result of tides raised by the Moon and Sun. To keep the time kept by rotation in synch with the constant-frequency atomic clock, we must add an occasional second, a "leap second." It is usually at the end of the year, and this will be a year for it. To the dismay of astronomers, this hybrid "Coordinated Universal Time" (UTC) is under fire from the frequency people, who would divorce timekeeping entirely from terrestrial rotation. It's a move that goes against the philosophical grain as well, as sunrise and sunset would gradually get out of synch with the clock.

Turbulence of motion extends downward deeply into the Earth, whose heat causes the internal convection that drives continental drift, makes volcanoes, and so on. Part of the heat is that left over from formation 4.5 billion years ago. The detection of antineutrinos from inside the Earth finally prove that the rest of the heat is continuously generated by the radioactive decay of uranium and thorium (a byproduct of which is helium).

The Solar Gazette

A massive solar flare (caused by reconnection of powerful magnetic fields) tossed protons directly at Earth, surprising theory, but not of course, the Sun. While out of order, this is a good point to mention that solar flares can be dwarfed by those that take place on M dwarf stars. Even L dwarfs exhibit such phenomena, even though such low mass stars should not have such strong magnetic fields. The best known flare star is our own Proxima Centauri, the closest star to Earth. A flare on Gliese 3685A momentarily increased the star's ultraviolet brightness 4000 times. If you vacation in space, don't go "red dwarf bathing."

Reversing direction back to our Solar System, one crucial goal of solar science was reached when Voyager I -- an astounding 94 Astronomical Units (AU) out -- passed the solar wind "termination shock," where the wind speed drops from supersonic to subsonic. The next step is the heliopause, where the wind slams into the interstellar medium.

Mostly Mars and Saturn

Which of the two made the biggest splash? Data continue to "pour in" on the subject of Martian water, including the discovery of beds of minerals that require the presence of the precious fluid (gypsum, magnesium sulfate), a possible "frozen sea" under Elysium, an ice-containing crater, branching networks, and so on. The only concern comes from lack of carbonates. The planet may also be more active than we once thought, cratering rates showing lava flowing from volcanoes a mere hundred or so million years ago.

The Martian spacecraft and rovers are rivalled by Saturn's Cassini and Huygens, which had imaged and studied rings and moons and actually sent a package down to Titan that revealed the terrain in fine detail, showing branching networks and "lakes" of some sort that might have held (and maybe still do hold) liquid hydrocarbons, though no obvious methane oceans and seas pop up. We do find high winds in the hundreds of miles per hour in the thick atmosphere, and evidence for methane, acetylene, and a variety of other compounds. Then there is Enceladus with its patchy light but wet atmosphere that may be associated with the young "tiger stripes" seen on the surface. They may be caused by icy volcanism associated with tidal flexure and heating. The list goes on with two-faced Iapetus, one side dark as a black velvet Elvis painting, the other side reflecting half the radiation that falls upon it, the dark side caused by the satellite sweeping up orbiting debris. Mountains 20 kilometers high defy reason.

Even from Earth, Saturn is glorious, Hubble clearly observing aurorae at the south pole, the one now turned toward us.

Moon score: Jupiter 63, Saturn 50. But watch this space.

Even Uranus gets into the act. As the highly tilted planet orbits, the overhead Sun is moving toward the equator, which is resulting in an increase in clouds and cloud bands. The planet has by far the greatest seasonal changes of any in the Solar System, its "arctic circle" at 8 degrees latitude, its "tropics" at 82 degrees.

Little Stuff

We are a big hit with the comets. Quite a destructive one at that, all on purpose when on July 4 "Deep Impact" slammed into Comet Tempel 1. Images taken just before impact revealed odd linear features and strange craters. The collision sprayed huge amounts of very fine ambient dust, and showed the comet to be held very loosely together, its density only half that of water. At the other end of the cometary spectrum is Hale-Bopp, which, surprisingly, is still active even though 21 AU from the Sun, farther than Uranus.

Asteroids at least try to strike back with yet another impact scare, this time from 2004 MN4. Rather than hitting, however, the asteroid will make a near miss on April 13, 2029. Brightening to third magnitude at a distance of 30,000 km, the best view will be in (of course) Europe.

Again reversing the situation, impacts on asteroids shake the little bodies so much that they smooth out, wiping out craters and causing landslides, thus explaining the odd terrain seen on Eros.

At the far end of the Solar System's disk, from Neptune to beyond, lies the debris of planet formation that makes the Kuiper Belt, the reservoir that sends us short-period comets. Some 1000 Kuiper Belt Objects (KBOs) have been found. The bodies of the Kuiper Belt were too thinly spread to have accumulated into a large planet. They seem to be brighter and therefore smaller than once thought, so the total mass of KBOs has gone down, to but a fraction that of Earth.

Not so Little: Planet or Not?

Though the Kuiper Belt seems to have a ragged outer edge around 50 to 55 AU, scattered bodies lurk farther away. And some of them are -- relatively speaking -- pretty big. Prior to this year, astronomers found KBOs that approached the size of Pluto, and then finally exceeded it with 2003 UB 313. At its current distance of 97 AU, it shines at but 19th magnitude, its elliptical orbit of 557 years taking it as close to the Sun as 38 AU. There may be many more such big guys, all being relative

The discovery (and just who discovered it is being contended) once again prompted the seemingly age-old question of "what is a planet anyway?" The discoverers would certainly rather have a planet in their bag than just the biggest KBO. If Pluto is a planet, why not 2003 UB 313? If "UB" is not, should we demote Pluto? As much as anything it is cultural: Pluto has been a "planet" for 75 years and kids like it.

More Dust

All of our Solar System came from the dusty gases of interstellar space, which the Spitzer Space Telescope is highly adept at observing. A fine dusty example is given by the dark lanes that divide the Trifid Nebula. While the dust's low temperature precludes visual radiation, it glows strongly in the infrared, where the lanes become bright, the IR radiation also revealing embedded new stars.

Planetary Disks

As stars condense out of the cosmic gloom and rotate ever faster (thanks to the conservation of angular momentum), they develop the dusty gaseous disks that ultimately condense into planets. While the original thick disks quickly disappear, a residual is left over that is continuously populated by dust from large-body collisions. Some stars with disks have clearly-defined planets, while others do not (and may simply be undetected).

Numerous such disks are seen around class A dwarfs, several of which have internal "holes." Rings in the 80-AU-radius gap inside the very large Beta Pictoris disk (10 times the size of ours) suggest an otherwise unseen planet at 12 AU from star. An asymmetrical hole in Fomalhaut's disk (which totals over 50 earth masses) may be caused by a planet at 50-70 AU out.

And More Planets

The exoplanet business is as booming as the KBO industry with 160 of the critters so far discovered, nearly all by detecting the Doppler shifts in the stellar spectra induced by the minute stellar motions caused by planetary gravity. Precision is now down (up?) to 1 meter per second! And with an increasingly longer time baseline, the results pour in. A sampling:

1. Gliese 876, an M4 dwarf in Aquarius 15.3 light years away, has a triple planet system. The outer two have masses comparable to that of Jupiter with periods of 61 and 30 days and are actually locked into a 2:1 orbit ratio. The innermost holds the mass record of a 9 Earth-mass planet just 0.021 AU out with an orbital period of just 1.9 days!

2. Instead of three planets, HD 188753 (7th magnitude in Cygnus 145 light years off) has three STARS. The planet (about a Jupiter mass, 0.045 AU orbit of 3.35 days) goes around a K0 star that also has a binary companion 12 AU out (but comes as close as 6 AU). How a planet could form and survive in such an environment is anyone's guess.

3. The same question holds for a planet in orbit about Gliese 86, a sixth magnitude K1 dwarf in Eridanus 36 light years away. Here a 4 Jupiter mass (minimum) planet with a period of 15.8 days lies 0.11 AU from the star, while 20 AU out is a white dwarf! How did the planet survive the giant phases that preceded the white dwarf state?

4. HD 149026 (an 8th magnitude G0 dwarf in Corona Borealis that is starting to evolve) has a 1.2 Saturn mass planet in a 2.88 day orbit 0.11 AU out that transits the star. The eclipse reveals a radius only 85 percent Saturn's, giving a density twice Saturn's, implying that the planet has a lot of "rock and ice" and is more like Uranus.

5. A 1/5-Pluto mass planet orbits a pulsar (B1257+12) that has three other planets already around it. (Such planets seem to condense from the debris of a one-time companion star that had been tidally destroyed by the pulsar.)

In addition to these, the radiation from two transiting close-in "hot Jupiters" (TrES-1 and HD 209458) has been detected, revealing temperatures in the neighborhood of 1100 Kelvin. Yet direct imaging of exoplanets is elusive. In one touted case, a "planet" seems to hover near a brown dwarf, yet it may just be a line-of- sight coincidence. The technology is just not yet there.

Brown Dwarfs

We can presumably recognize brown dwarfs -- substars below 0.075 solar masses that cannot run the full proton-proton fusion chain -- by their temperatures, luminosities, and an application of theory. Yet AB Doradus shows we may not know as much as we think. While theory says that AB Dor C (which goes around AB Dor A) is a brown dwarf, orbital analysis shows it to be a low mass M dwarf with double the theoretically expected mass. Still, no one doubts that such substars are all over the place.

Real Stars

Climbing back up the mass ladder, we look for solar behavior in other stars. We find it in Kappa-1 Ceti, for which the Canadian high-precision photometric MOST satellite (which bears a striking resemblance to Sponge Bob) has detected variations (from starspots moving in and out of sight) that for the first time reveal differential rotation in a star other than the Sun (in which higher latitudes have longer rotation periods than lower ones).

Cetus also gives us the famed long-period variable star Mira, in which mass is now seen to flow from the giant star to the white dwarf companion. R Aquarii is in a similar situation, the system surrounded by a nebula made of matter lost by the evolving giant. Old Korean records show a nova (novae caused by runaway thermonuclear explosions on mass-accreting white dwarfs) at that location around 1074. Perhaps Mira will produce one too.

While the low-mass limit to stars is pretty secure, the upper limit -- which is usually taken as 120 or so solar masses -- is not. Studies of young open clusters (which still have their high mass stars intact) now clearly show that the most massive are in the 120 to 150 range, supporting the common wisdom. The Galaxy still makes such clusters, by the way. Newly-discovered Westerlund-1, 15,000 light years away and only 4 to 5 million years old, contains in excess of 100,000 stars.

As Mira evolves and loses its outer envelope, it is expected to produce a fine planetary nebula (PN). Though studied intensively, we still do not understand PN structures all that well. Our ignorance is exemplified by the Cat's Eye Nebula, NGC 6543 (in which Herschel discovered central stars and through which Huggins found that PN are gaseous), which has an enormously complex structure in which we see rings from episodic mass loss. Looking back to the past, stars become metal poor. The Big Bang created only hydrogen, helium, and a bit of lithium. The first stars ("Population III", Pop II the low-metal stars of the Galactic halo, Pop I the higher metal stars of the disk) should be metal free. Though we cannot find Pop III, we can get mighty close. A new record has been set with a star with a quarter-millionth the solar iron-to-hydrogen ratio. Such stars reveal an abundance pattern (that of rapid neutron capture) produced by supernova explosions, supporting the idea that the first hydrogen-helium stars were massive and quickly blew up such that Pop III no longer exists.

Supernovae

Which brings us to this most violent of stellar acts. The supernova encompasses four areas of study, the explosion itself, the gaseous expanding "supernova remnant," and two possible end products: neutron stars and black holes. The latter two objects are produced by high-mass core collapse supernovae. Another kind of supernova ("Type Ia") is produced by the flow of matter onto a white dwarf from a companion that causes the white dwarf to exceed its support limit of 1.4 solar masses. Collapsing and exploding, Type Ia supernovae leave no dense remnants behind.

The Whirlpool Galaxy, M 51, gave us a nifty core-collapse supernova whose progenitor was identified as a 7-11 solar mass red supergiant, allowing a finer lower limit to masses of the stars that produce core-collapse explosions (one close to the 8-10 solar mass theoretical limit).

While Supernova 1987A was clearly a core-collapse event, there is still no sign of a pulsar/neutron star. Such pulsars span a wide range. At the top are "magnetars," pulsars with magnetic fields some 100 trillion times stronger than that of Earth and that seem to be caused by the collapse of the most massive stars. Internal adjustments make the neutron stars into "soft gamma ray repeaters" (SGRs), whose intense bursts of radiation can affect the Earth's atmosphere and orbiting satellites. We recently witnessed the most energetic "superflare" yet seen, from SGR 1806-20, which is on the other side of the Galaxy, 50,000 light years from here.

Even "normal" pulsars can be weird. Old pulsars that have lost their spins can be "spun up" by mass accreted from a companion until they spin hundreds of times per second (the destruction of the companions leading to the "pulsar planets" discussed above). Globular clusters are particularly rich in these "millisecond pulsars," as the star density promotes duplicity. The globular Terzan 5 sets the record with 200 of them. The steady disruption of globular clusters (mostly by tides raised by the Galaxy) may provide many, if not all, of the millisecond pulsars in the Galaxy's general field.

The more massive stars may be the progenitors of stellar black holes, though nobody knows (massive stars seeming to make the magnetars). Among the most famous of compact collapsed objects is the one in SS 433, which is related to the supernova remnant W 50. While it was long thought to be a neutron star, a new mass measurement puts it above the limit at which neutron stars can support themselves, revealing that it may well be a black hole, adding one more to the potential pile.

Tycho's star appears to have been a Type Ia event. The explosion apparently sent the donor star screaming off as "Tycho G," a sunlike dwarf that is moving away at a speed much higher than other stars in the neighborhood. As an aside, that's nothing compared to a new Galactic stellar speed record of 850 km/s relative to the Sun, 55 times normal, and possibly caused by an encounter between the star and a black hole. That's more than enough to eject the star from the Galaxy.

The expanding supernova remnant carries heavy elements made in the explosion back into the cosmos. Shock waves in the Crab nebula have apparently destroyed the fine dust particles that are created and ejected as well, leaving only larger ones. The shocks are suspected to accelerate atomic nuclei (mostly protons) and electrons to speeds approaching that of light, whence they become "cosmic rays." These particles (which are NOT electromagnetic radiation!) steadily rain on Earth, collisions with atoms producing the carbon-14 that is used in radioactive dating, showing the interconnectedness of it all. Energies are astounding. At maximum, a single atomic nucleus can have the energy of a professionally pitched baseball.

Gamma Ray Bursts

Yet there is a growing sense that most cosmic rays may be created in the "hypernova" explosions of the most massive stars, the ones that produce black holes (or maybe magnetars) and gamma ray bursts. About once a day we observe such intense bursts of gamma radiation (the most energetic kind of "light") coming from deep in the cosmos, from "galaxies far far away." Of the two kinds, "long" and "short," the long bursts are the ones associated with the hypernovae. The short bursts are believed to be caused by the mergers of binary neutron stars or neutron stars with black holes. (They close in on each other as a result of gravitational radiation.) This concept was boosted by the observation of a short burst from an elliptical galaxy, one devoid of the massive stars that make the long bursts.

The intensity of the long bursts is in part the result of relativistic "beaming," as the explosion comes out in twin bipolar flows. If the beam is not pointed at us we see nothing. Jets in the Cassiopeia A supernova remnant (from an explosion around 1680) suggest that the same energetic phenomena occur here (and we are back to cosmic rays). A prime candidate is Eta Carinae, a massive double star set against the Carina Nebula, the more massive of which may be close to blowing up. Though 8000 or so light years away, if its beam were pointed toward Earth, it could do serious damage to life. Fortunately, the rotation axis is pointed elsewhere.

The Galaxy

All these various stars are collected into our spiral Galaxy, a new map of which has recently been released, showing arm locations everywhere except for a hidden portion on the other side from us. Beryllium observations in stars of a globular cluster reveal that the cluster formed only a couple hundred million years after the birth of the Galaxy. Given the calculated age of the cluster, the age of the Galaxy is then consistent with the age of the Universe (that is, just a bit younger) as derived from cosmological principles (13.7 billion years). Dating by stellar thorium content gives an age to the thin disk of 9 billion years, as expected.

At the center of the Galaxy is a supermassive black hole of around three million solar masses. Chandra X-ray observations suggest that thousands of stellar-mass (in the tens of solar masses) black holes surround it. Such black holes are "visible" when they accrete matter from companions and occasionally violently flare up. In any collection of stars, the heavier ones will sink to the center, the phenomenon apparent in both open and globular clusters. The collection of black holes at the center may have accumulated there from all over the Galaxy.

Galaxies

While our Galaxy's globular clusters formed early on, one of ours (Whiting 1) is a mere five billion years old, and probably came to us through merger. A look at our Galaxy's surroundings reveals numerous shredded dwarf galaxies associated with star streams that are being tidally destroyed by our own Galaxy and will merge with us. Some are incredibly small. The newly discovered Ursa Major dwarf (350,000 light years off), almost impossible to see against the stellar foreground, has a luminosity of only 40,000 Suns, far less than our most luminous stars. These discoveries support the idea that galaxies in general were built by mergers from smaller systems.

A bit farther out, M 33 in Triangulum gave up its proper motion (angular motion across the sky), the first such galaxy to do so, leading the way toward a real understanding of motions with the Local Group of galaxies. Nearby M 31, while always seeming large, just became even bigger. Recent observations show that it may extend a quarter of the way to us. Its "double nucleus" appears to be a massive black hole that is surrounded by an irregular orbiting disk of stars.

Through the joining of small images, Hubble produced an extraordinary mosaic image of M 51 (the Whirlpool) and its companion, while Spitzer gave us an infrared-radiating BRIGHT disk in M 104 (the Sombrero). To complete this small story of individual galaxies, spiral arms have finally been seen in M 82 in Ursa Major, an oddball whose central regions are spewing gas perpendicular to the disk as a result of numerous supernova explosions.

Galaxies tend to gang into clusters, the more massive of which are filled with X-ray emitting hot gas (which makes up several percent of the mass of the Universe). Collisions between clusters are the most energetic events known, and are another possible source of high-energy cosmic rays, which connects us not just with the stars of our Galaxy but with the distant cosmos as well.

At greater distances lie the quasars. Because of light travel time, we see them younger than our own surroundings. Quasars (QSOs) are massive black holes at the centers of young galaxies that are rapidly accreting matter from their surroundings and are therefore bright (our own nucleus a sort of lower mass dead quasar). Yet Hubble finds a naked quasar. Hmmm.

DM 'n DE (Dark Matter and Dark Energy)

Together "DM" and "DE" constitute 95 percent of the mass of the Universe (respectively 25 and 70). The rest is in ordinary matter, almost all in extragalactic gas (stars making well under 1 percent). DM and DE are, however, still "dark" as we have little idea of what constitute them. At least we now know through Chandra observations, and through studies of galaxy collisions, that elliptical galaxies can, and do, have their share of dark matter. (Chandra revealed hot gas that can only be gravitationally held by large quantities of DM. And DM influences the behavior of galaxy collisions.)

And At Last, Everything

Well, almost. One observation suggested a redshift-10 galaxy (made visible by gravitational lensing) that must have been created just 500 million years after the Big Bang. Others suggest not. And so it goes. Yet Hubble still finds evidence for faint red galaxies at redshifts 6 to 8, small ones a tenth the size of ours (remember the mergers).

The "Big Bang" is metaphorical, as it was not an explosion, but a sudden expansion of spacetime, the origin of which is unknown. Yet there is evidence that hundreds of thousands of years later, as light was able to escape matter (to produce the Cosmic Background Radiation), acoustic waves where indeed produced and that they played a role in the observed filamentary distribution of galaxies that we see today.

At an age of 13.6 billion years, expanding locally at about 70 kilometers per second per megaparsec (the parsec 3.26 light years), accelerating in expansion speed as a result of "DE," the Universe at large seems all so distant and impersonal. Yet without it all, we would not be here. From the Big Bang to Butterflies, it took all of it to make us...and to allow its appreciation even if we cannot fully understand it.