Galaxies

Click the icon to view galaxies of the Messier catalog

>>Links; >>References; >>Galaxy Clusters

The icon shows M51, the Whirlpool Galaxy.

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Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebulae.

We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant, while the nearest giant neighbor, the Andromeda Galaxy, also a spiral, is about 2-3 million light years distant.

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Spiral

Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddish diffuse emission nebulae, or as dark dust clouds) and young (open) star clusters and associations, which have emerged from them (recognizable from the blueish light of their hottest, short-living, most massive stars), often arranged in conspicuous and striking spiral patterns and/or bar structures, and an ellipsoidally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters. The young stars in the disk are classified as stellar population I, the old bulge stars as population II. The luminosity and mass relation of these components seem to vary in a wide range, giving rise to a classification scheme. The pattern structures in the disk are most probably transient phenomena only, caused by gravitational interaction with neighboring galaxies.

Our sun is one of several 100 billion stars in a spiral galaxy, the Milky Way.

Lenticular (S0)

These are, in short, "spiral galaxies without spiral structure", i.e. smooth disk galaxies, where stellar formation has stopped long ago, because the interstellar matter was used up. Therefore, they consist of old population II stars only, or at least chiefly. From their appearance and stellar contents, they can often hardly be distinguished from ellipticals observationally.

Elliptical

Elliptical galaxies are actually of ellipsoidal shape, and it is now quite safe from observation that they are usually triaxial (cosmic footballs, as Paul Murdin, David Allen, and David Malin put it). They have little or no global angular momentum, i.e. do not rotate as a whole (of course, the stars still orbit the centers of these galaxies, but the orbits are statistically oriented so that only little net orbital angular momentum sums up). Normally, elliptical galaxies contain very little or no interstellar matter, and consist of old population II stars only: They appear like luminous bulges of spirals, without a disk component.

However, for some ellipticals, small disk components have been discovered, so that they may be representatives of one end of a common scheme of galaxy forms which includes the disk galaxies.

Irregular

Often due to distortion by the gravitation of their intergalactic neighbors, these galaxies do not fit well into the scheme of disks and ellipsoids, but exhibit peculiar shapes. A subclass of distorted disks is however frequently occuring.

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The first known galaxies were longly known before their nature as "island universes" came to light - this fact was finally proven only in 1923 by Edwin Powell Hubble, when he found Cepheid variable stars in the Andromeda Galaxy M31. Ancient observers have known the Milky Way and - on the Southern Hemisphere - the Large and the Small Magellanic Cloud since prehistoric times, and there are speculations that also the Andromeda Galaxy M31 may have been observed and recorded as a nebulous patch by anonymous Babylonian observers around 1,300 B.C.. This object was certainly known to medevial Persian astronomers before 905 A.D., and cataloged and described by Persian astronomer Al Sufi in 964 A.D. All other galaxies have been discovered only after the invention of the telescope: The Triangulum Galaxy M33 was first seen by Italian Priest astronomer G.B. Hodierna before 1654. Next, French astronomer Legentil discovered M32, a companion of the Andromeda Galaxy, in 1749, and his compatriot Abbé Lacaille found M83 in 1752, the first galaxy beyond the Local Group to be discovered. These six were all external galaxies to be known, before Charles Messier started to survey the sky for comets and "nebulae." His first original discovery of a galaxy, M49, a giant elliptical member of the Virgo Cluster, occurred in 1771. The Messier Catalog in his modern form contains 40 galaxies, all but the two Magellanic Clouds that have been found up to 1782. Starting in 1783, William Herschel found and cataloged over 2,500 star clusters and "nebulae" up to 1802, 2,143 of them actually galaxies. J.L.E. Dreyer's NGC catalog contains 6,029 (about 75.9%), and his IC catalog another 3,971 galaxies (about 73.7%).

From their appearance, galaxies are classified in types as given above, as spiral, lenticular, elliptical, and irregular galaxies, where spirals may be further classified for the presence of a bar (S: spirals, SAB: Intermediate, SB: Barred spirals). More precisely, ellipticals are sub-classified for ellipticity from E7 (strongly elongated) to E0 (circular), and spirals for prominence of bulge versus spiral arms from Sa (or SABa, SBa) to Sc or Sd. This so-called Hubble Classification Scheme can well be illustrated by Messier's galaxies:

• Hubble Scheme with Messier galaxies for HTML browser supporting tables (e.g., Netscape, IBM Webexplorer)
• Hubble Scheme for HTML browsers not supporting tables (Lynx, Mosaic)

Galaxies of all types, though of a wide variety of shapes and appearances, have many basic common features. They are huge agglomerations of stars like our Sun, counting several millions to several trillions. Most of the stars are not lonely in space like our Sun, but occur in pairs (binaries) or multiple systems.

The most massive galaxies are giants which are a million times more massive than the lightest: Their mass range is from at most some million times that of our Sun in case of the smallest dwarfs, to several trillion solar masses in case of giants like M87 or M77. Accordingly, the number of stars in them varies in the same range.

The linear size of galaxies also scatters, ranging from small dwarfs of few thousands of light years diameter (like M32) to respectable several 100,000 light years. Among the biggest Messier galaxies are the Andromeda galaxy M31 and the bright active Seyfert II galaxy M77.

Our Milky Way Galaxy, a spiral galaxy, is among the massive and big galaxies with at least 250 billion solar masses (there are hints that the total mass may even be as large as 750 billion to 1 trillion times that of the Sun) and a disk diameter of 100,000 light years.

Besides very many individual stars, most galaxies contain the following typical objects:

• Globular star clusters, large but quite compact agglomerations of some 100,000 to several million stars. These large clusters have about the same mass as the smallest galaxies, and are among the oldest objects in galaxies. Often, they form conspicuous systems, and occur at galaxies of every type and size. The globular cluster systems vary in a wide range in richness between the individual galaxies.
• As the stars develop, many of them leave nebulous remnants (planetary nebulae or supernova remnants) which then populate the galaxies.
• While the older stars, including the globular clusters, tend to form an ellipsoidal bulge, the interstellar gas and dust tends to accumulate in clouds near an equatorial disk, which is often conspicuous (i.e., in spiral and lenticular galaxies).
• The interstellar clouds are the places of star formation. More acurately, huge diffuse star-forming nebulae are places where crowded (open) clusters and associations of stars are formed.
• A rather dense galactic nucleus, which is somewhat similar to a "superlarge" globular cluster. In many cases, galactic nuclei contain supermassive dark objects, which are often considered as Black Hole candidates. Some of the more massive and conspicuous globulars are suspected to be the remnants of former nuclei of small galaxies which have been disrupted and cannibalized by larger galaxies.

Galaxies normally emit light of every wavelength, from the long radio and microwave end over the IR, visual and UV light to the short, high-enregy X- and gamma rays. Interstellar matter is coolest and therefore best visible in radio and IR, while supernova remnants are most conspicuous in the high-energy part of the electromagnetic spectrum.

Some galactic nuclei are remarkably distinguished from the average: These so-called Active Galactic Nuclei (AGNs) are intensive sources of light of all wavelengths from radio to X-rays. The activities seen in the AGNs are caused by gaseous matter falling into, and interacting with, the supermassive central objects mentioned above, according to the current consensus of most researchers. Sometimes, the spectra of these nuclei indicate enormous gaseous masses in rapid motion; galaxies with such a nucleus are called Seyfert galaxies (for their discoverer, Karl Seyfert). M77 is the brightest Seyfert galaxy in the sky. Few galaxies have even more exotic nuclei, which are extremely compact and extremely bright, outshining their whole parent galaxy; these are called quasars (an acronym for QUAsi-StellAR objects). From their properties, quasars resemble extremely active Seyfert galaxy nuclei. However, quasars are so rare and the nearest is so remote that the brightest of them, 3C273, about 2 billion lightyears away in the constellation Virgo, is only of magnitude 13.7, and none of them is in Messier's or even in the NGC or IC catalog.

Occasionally, at irregular intervals given by chance, in any type of galaxies, a supernova occurs: This is a star suddenly brightning to a high luminosity which may well outshine the whole galaxy; the maximal absolute magnitude of a supernova may well reach -19 to -20 magnitudes. This remarkable phenomenon has attracted the attention of many astronomers (equally both professionals and amateurs), who observe galaxies regularly as they "hunt" supernovae. Supernovae have been observed in several Messier catalog galaxies.

According to our current scientific understanding, at least most galaxies (including our Milky Way and those in Messier's catalog) have formed during a comparatively short period, at about the same time, within the first billion years after the universe started to expand, from an initial hot state. Thus they are all almost as old as the universe itself, currently thought to be about 10-15 billion years. It is thought that galaxy formation started when primordial clouds of gaseous matter (hydrogen and helium), the proto-galaxies, were singled out and started to collapse by their own gravity. According to computer simulations, the variety of galaxy forms results from different initial parameters of the proto-galaxies such as the amount of (initial) angular momentum, as well as their later evolution in their environments, such as interaction with other neighboring galaxies.

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Messier's galaxies are not deitributed equally across the sky, but can be grouped into a large group of Northern Spring/Southern Fall, and a smaller Northern Fall/Southern Spring group:

• Northern Fall/Southern Spring galaxies (6):
  • Local Group members (4): M31 group (M31, M32, M110), and M33.
  • Other Northern Fall/Southern Spring galaxies (2): M74, M77.
• Northern Spring/Southern Fall galaxies (34):
  • Virgo Cluster Galaxies (16): M49, M58, M59, M60, M61, M84, M85, M86, M87, M88, M89, M90, M91, M98, M99, M100.
  • Galaxies in Leo (5): M66 group (M65, M66), M96 group (M95, M96, M105).
  • Galaxies in and around Ursa Major (11): M81 group (M81, M82), M101, M102, M108, M109, M51 group (M51, M63), M94, M106, M64.
  • Southern galaxies (2): M83, M104.

In the regions between, there are RA ranges without any Messier galaxies (3-8 and 16-23h in RA); these include the regions of the Milky Way band of stars which obscures the background galaxies.

MARS

The Bringer of War

Mars is the fourth planet from the Sun and the seventh largest:

      orbit: 227,940,000 km (1.52 AU) from Sun
     diameter: 6,794 km

     mass: 6.4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this symbolism.) The name of the month March derives from Mars.

Mars has been known since prehistoric times. Of course, it has been extensively studied with ground-based observatories. But even very large telescopes find Mars a difficult target, it's just too small. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

Viking 2 Landing Site

Pathfinder Landing Site

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures; they are still operating after more than a year on Mars. Three Mars orbiters (Mars Global Surveyor, Mars Odyssey, and Mars Express) are also currently in operation.

Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.

Olympus Mons

Mars has some of the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:

• Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.
• Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.
• Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep (top of page);
• Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. (None of this is visible in any detail with a telescope, even the Hubble Space Telescope; all this information comes from the spacecraft that we've sent to Mars.)

Southern Highlands

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' accretion). Mars Global Surveyor has produced a nice 3D map of Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide).

Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity. However, data from Mars Global Surveyor indicates that Mars very likely did have tectonic activity sometime in the past.

Valley Network

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid on the surface. Liquid water is the obvious fluid but other possibilities exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice images of layered terrain taken by Mars Global Surveyor and the mineralology results from MER Opportunity. Most of these point to wet episodes that occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. However, images from Mars Express released in early 2005 show what appears to be a frozen sea that was liquid very recently (maybe 5 million years ago). Confirmation of this interpretation would be a very big deal indeed! (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

South Polar Cap

Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA's Mars Express has shown that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Mars by HST

Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers (more details from STScI).

The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars.

A small number of meteorites (the SNC meteorites) are believed to have originated on Mars.

On 1996 Aug 6, David McKay et al announced what they thought might be evidence of ancient Martian microorganisms in the meteorite ALH84001. Though there is still some controversy, the majority of the scientific community has not accepted this conclusion. If there is or was life on Mars, we still haven't found it.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars' interior and for the past history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible with the unaided eye. Mars is a difficult but rewarding target for an amateur telescope though only for the three or four months each martian year when it is closest to Earth. Its apparent size and brightness varies greatly according to its relative position to the Earth. There are several Web sites that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mars' Satellites

Mars has two tiny satellites which orbit very close to the martian surface:
Distance Radius Mass Satellite (000 km) (km) (kg) Discoverer Date --------- -------- ------ ------- ---------- ---- Phobos 9 11 1.08e16 Hall 1877 Deimos 23 6 1.80e15 Hall 1877 ("Distance" is measured from the center of Mars).

Death Echos of Material Destroyed Near a Black Hole

Death Echos of Material Destroyed Near a Black Hole

Greedy black holes can only consume so much material. The leftover matter backs up into an accretion disk surrounding the black hole. The pull of the black hole is so strong that flashes of radiation emitted from this accretion disk might need to make several orbits around the black hole before it can actually escape the gravitational pull. And these echoes might serve as a probe, allowing astronomers to understand the nature of the black hole itself.

Keigo Fukumura and Demosthenes Kazanas from NASA's Goddard Space Flight Center revealed their theoretical research at the Winter meeting of the American Astronomical Society.

"The light echoes come about because of the severe warping of spacetime predicted by Einstein," said Fukumura. "If the black hole is spinning fast, it can literally drag the surrounding space, and this can produce some wild special effects."

Black holes are surrounded by a disk of searing hot gas rotating at close to the speed of light. A black hole can only consume material so quickly, so any additional matter backs up into this accretion disk. The material in these disks can form hot spots which emit random bursts of X-rays.

When the researchers accounted for the predictions made by Einstein's general theory of relativity, they realized that the severe warp of spacetime can actually change the path X-rays take as they escape the grasp of the black hole. The X-rays can actually be delayed, depending on the position of the black hole, the position of the flare, and Earth.

If the black hole is rotating at the most extreme speeds, photons can actually make several orbits around the black hole before escaping.

"For each X-ray burst from a hot spot, the observer will receive two or more flashes separated by a constant interval, so even a signal made up from a totally random collection of bursts from hot spots at different positions will contain an echo of itself," says Kazanas.

Astronomers watching these flashes will have a powerful observational tool they can use to probe the nature of the black hole. The frequency of the flashes would provide astronomers with an accurate way to measure the mass of the black hole.

Original Source: NASA News Release

Using Gravity to Find Planets in the Habitable Zone

Astronomers have several techniques to discover planets. But one of the least used so far, gravitational microlensing, might be just the right technique to find planets in the habitable zone of nearby dwarf stars.

The first way astronomers find planets is with the radial velocity technique. This is where the gravity of a heavy planet yanks its parent star around so that the wobbling motion too and fro can be measured.

The second technique is through transits. This is where a planet dims the light coming from its parent star as it passes in front. By subtracting the light from when the planet isn't in front of the star, astronomers can even measure its atmosphere.

The third way is through gravitational microlensing. When two stars are perfectly lined up, the closer star acts as a natural lens, brightening the light from the more distant star. Here on Earth, we see a star brighten in a very characteristic way, and then dim down again. A blip in the change of brightness can be attributed to a planet.

Unlike the other two methods, microlensing allows you to reach out and see planets at tremendous distances - even clear across the galaxy. The problem with microlensing is that it's a one-time opportunity. You're never going to see those stars line up in just the same way again.

But Rosanne Di Stefano and Christopher Night from the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA think there's another way microlensing could be used. In their research paper entitled, Discovery and Study ofNearby Habitable Planets with Mesolensing, the researchers propose that many stars have a high probability of becoming a lens.

Instead of watching the sky, hoping to see a lensing event, you watch specific stars and wait for them to pass in front of a more distant star.

These high-probablility lenses are known as mesolenses. By studying a large number of dwarf stars, they expect that many of them should pass in front of a more distant star as often as once a year. And if pick your targets carefully, like dwarf stars moving in front of the Magellanic Clouds, you might get even more opportunities.

Unlike other methods of planet detection, gravitational lensing relies on light from a more distant star. It is therefore important to ask what fraction of nearby dwarfs will pass in front of bright sources and so can be studied with lensing. Within 50 pc, there are approximately 2 dwarf stars, primarily M dwarfs, per square degree.

For less massive red dwarf stars, you should be able to see them at a distance of 30 light years, and for Sun-mass stars out to a distance of 3,000 light years. These stars are close enough that if a planet is detected in the habitable zone, followup techniques should be possible to confirm the discovery.

They calculated that there are approximately 200 dwarf stars passing in front of the Magellanic Clouds right now. And many of these will have lensing events with the stars in the dwarf galaxies.

Instead of monitoring specific stars, previous surveys have just watched tens of millions of stars per night - hoping for any kind of lensing event. Even though 3,500 microlensing candidates have been discovered so far, they tend to be with stars at extreme ranges. Even if there were planets there, they wouldn't show up in the observations.

But if you pick your stars carefully, and then watch them for lensing events, the researchers believe you should see that brightening on a regular basis. You could even see the same star brighten several times, and make follow-up observations on its planets.

And there's another advantage. Both the radial velocity and transit methods rely on the planet and star being perfectly lined up from our vantage point. But a microlensing event still works, even if the planetary system is seen face on.

By using this technique, the researchers think that astronomers should turn up lensing events on a regular basis. Some of these stars will have planets, and some of these planets will be in their star's habitable zone.

GLOBAL WARMING

You weren't imagining things, 2007 really was an unseasonably hot year. In fact, it was tied with 1998 for the second hottest year on record. All in all, the 8 warmest years have all occurred since 1998, and the 14 warmest years since 1990. This mini-record was announced by NASA climatologists this week.

Researchers from NASA's Goddard Institute for Space Studies used temperature data from weather stations on land, satellite measurements of sea ice temperatures since 1982 and data from ships for earlier years.

"As we predicted last year, 2007 was warmer than 2006, continuing the strong warming trend of the past 30 years that has been confidently attributed to the effect of increasing human-made greenhouse gases," said James Hansen, director of NASA GISS.

Perhaps the most warming occurred up in the Arctic and high latitude regions of the planet, where vast regions of ice melted away. In fact, the Northwest Passage opened up for the first time, and scientists are predicting that the region could be ice free in the Summer in less than a decade.

The lower ice levels in the Arctic provides more open water and reduces the amount of sunlight reflected back into space. This is expected to increase the rate of warming.

Let's hope 2008 isn't so hot.