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Positrons, or "anti-electrons," are anti-particles with the same mass as an electron but with opposite charge. The generation of energetic electron-positron pairs is common in extreme astrophysical environments associated with the rapid collapse of stars and the formation of black holes. These pairs eventually radiate their energy, producing extremely bright bursts of gamma rays. Gamma Ray Bursts (GRBs) are the brightest electromagnetic events known to occur, but the mechanism of how GRBs are produced is still a mystery. Using ever more energetic lasers, Lawrence Livermore researchers have produced a record high number of electron-positron pairs, opening exciting opportunities to study extreme astrophysical processes like GRBs in a laboratory environment.

Predicted by Albert Einstein in 1916 as a consequence of his theory of General Relativity, gravitational waves are ripples in the fabric of space and time produced by violent events in the universe. Gravitational waves are emitted by accelerating masses in much the same way as radio waves are produced by accelerating charges. As gravitational waves travel to Earth, these ripples in the space-time fabric bring with them information about their violent origins and about the nature of gravity that cannot be obtained by other astronomical tools. Working with funding from the NSF, researchers at CalTech and MIT have designed and built the Advanced Laser Gravitational Wave Observatory in Richland, Washington with the goal of observing and recording gravitational waves for the first time. The instrumentation is sensitive enough to detect a gravitational wave-induced change equivalent to one-ten-thousandth the size of a proton diameter.

One of the most important scientific discoveries of the 20th century was that the spectacular spiral galaxies, such as our own Milky Way, rotate much faster than expected, powered by an extra invisible gravitational force. For lack of a better term (coupled with a lack of better understanding), this mysterious force was dubbed "Dark Matter." However, dark matter models do not come out naturally and require some disturbing fine-tuning to explain the observations. For this reason, some scientists suggest that rather than being due to dark matter, the faster than expected rotation of galaxies may be due to Newton's law of gravity becoming progressively less accurate at large distances. Remarkably, decades after this alternative theory (a universe without dark matter) was proposed, it still cannot be conclusively ruled out.

Scientists using the Hubble Space Telescope have discovered that the immense halo of gas enveloping the Andromeda galaxy, our nearest massive galactic neighbor, is about six times larger and 1,000 times more massive than previously measured. The dark, nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our own Milky Way galaxy. If the Milky Way has a similarly huge halo, then the two galaxies' halos may be nearly touching already and quiescently merging long before the two massive galaxies collide.

NASA's MESSENGER mission to the planet Mercury came to a planned, but nonetheless dramatic, end on April 30, 2015 when it slammed into the planet's surface at about 8,750 mph, creating a new crater. MESSENGER was launched on Aug. 3, 2004, and began orbiting Mercury on March 17, 2011. Although it completed its primary science objectives by March 2012, the spacecraft's mission was extended two times, allowing it to capture images and information about the planet in unprecedented detail. Among its many accomplishments, the MESSENGER mission determined Mercury's surface composition, revealed its geological history, discovered that its internal magnetic field is offset from the planet's center, and verified that its polar deposits are dominantly water ice.

A traditional Chilean stone laying ceremony to celebrate the beginning of construction of the Large Synoptic Survey Telescope (LSST) was held on April 14, 2015 on a mountain peak in northern Chile. This marked the start of construction of the LSST, an 8 meter wide-field survey telescope that will image the entire visible sky a few times each week and provide an unprecedented amount of immediate information to the scientific community on a daily basis. The LSST is expected to see first light in 2019 and begin full operation in 2022.

NASA/ESA's Hubble Space Telescope celebrates its 25th year of service on April 24, 2015. During its 25 year long mission the Hubble Space Telescope has changed our view of the Universe significantly. Some of the most ground breaking discoveries made in astronomy in the 20th century were made by Hubble. There are a lot interesting facts that the average Hubble fan may not know about this famous telescope. For example, NASA named the world's first space-based optical telescope after American astronomer Edwin P. Hubble (1889 - 1953). Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the Big Bang theory. Another interesting fact -- In order to take images of distant objects, Hubble must be extremely steady and accurate. The telescope is able to lock onto a target without deviating more than 7/1000th of an arc second, or about the width of a human hair seen at a distance of 1 mile.

A University of Arizona-led team of astronomers has discovered that Type Ia Supernovae, which have been considered so uniform that cosmologists have used them as cosmic "beacons" to measure the depths of the universe, are not so constant after all. This discovery casts new light on the currently accepted view that the universe is expanding at a faster and faster rate, pulled apart by a poorly understood force called Dark Energy. The results have implications for big cosmological questions, including the possibility that the acceleration of the expanding universe may not be quite as fast as today's textbooks say.

Mars has distinct polar ice caps, but Mars also has belts of glaciers at its central latitudes in both the southern and northern hemispheres. A thick layer of dust covers the glaciers, so they appear as surface of the ground. But radar measurements show that underneath the dust there are glaciers composed of frozen water. New studies have now calculated the size of the glaciers and thus the amount of water in the glaciers. It is the equivalent of all of Mars being covered by more than one meter of ice.

Global Astronomy Month, organized each April by Astronomers Without Borders, is the world's largest annual global celebration of astronomy. Astronomers Without Borders connects people worldwide through innovative programs that are accessible to everyone regardless of geography and culture. Combining local events with online technology and a global community, Astronomers Without Borders is a leader in promoting understanding and peaceful international relations, while also supporting outreach and education in astronomy. Astronomers Without Borders' motto is "One People, One Sky."

Tracking the rotation speed of solid planets, like the Earth and Mars, is a relatively simple task -- Just measure the time it takes for a surface feature to roll into view again. But giant gas planets Jupiter and Saturn are more problematic for planetary scientists, as they both lack measurable solid surfaces and are covered by thick layers of clouds, foiling direct visual measurements by space probes. Saturn has presented an even greater challenge to scientists, as different parts of this sweltering ball of hydrogen and helium are known to rotate at different speeds, whereas its rotation axis and magnetic pole are aligned. A new method devised by researchers at Tel Aviv University proposes a new determination of Saturn's rotation period and offers insight into the internal structure of the planet. The researchers also measured the rotation of Jupiter and hope to apply their method to other gaseous planets in the solar system such as Uranus and Neptune.

Some of the greatest astronomers of the 1600s, including Hevelius - the father of lunar cartography - and Cassini, carefully documented the appearance of a new star in the skies in 1670. Astronomers know it today by the name Nova Vul 1670. Now, new discoveries made by a team from the Max Planck Institut fur Radioastronomie in Bonn, Germany, reveal that this "new" star that astronomers saw in the sky in 1670 was not a nova, but rather a much rarer stellar collision. Using modern sub-millimeter telescopes, the team was able to finally unravel the mystery more than 340 years later by analyzing a fascinating variety of molecules in the stellar emissions that provided the required tell-tale evidence.

The GK Persei Nova became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a Classical Nova -- an outburst produced by a thermonuclear explosion on the surface of a white dwarf star. Chandra first observed GK Persei in February 2000 and then again in November 2013. This 13-year baseline has provided astronomers with enough time to notice important differences in the X-ray emission and its properties. These new observations are helping astronomers develop a better understanding of the births, lives, and deaths of stars and how they interact with their surroundings.

Saturday is "Pi" Day. It is March 14 of the year 2015, or 3-14-15 -- the first five digits of the constant "Pi." At 9:26 AM and 53 seconds, "Pi" will be represented to 10 digits -- 3.141592653. The Greek letter "Pi" is the symbol used in mathematics to represent the ratio of the circumference of a circle to its diameter, which is approximately 3.141592653... Pi has been calculated to over 12 trillion digits beyond its decimal point. As an irrational and transcendental number, it continues infinitely without repetition or discernible pattern. So, to all of our AstroMart friends around the world, Happy "Pi" Day.

Nearly 500 million miles from the Sun lies a moon orbiting Jupiter that is slightly larger than the planet Mercury and may contain more water than all of Earth's oceans. Temperatures are so cold, though, that water on the surface of Ganymede freezes as hard as rock and the ocean lies roughly 100 miles below the crust. Nevertheless, where there is water there could be life as we know it. Though the presence of an ocean on Ganymede has been long predicted based on theoretical models, NASA's Hubble Space Telescope has now found the best evidence for it to date.