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Fall Sp Target Information FALL SP TARGET INFORMATION MESSIER 13 BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Hercules BEST VIEW: Late July DISCOVERY: Edmond Halley, 1714 DISTANCE: 25,100 ly DIAMETER: 145 ly APPARENT MAGNITUDE: +5.8 APPARENT DIMENSIONS: 20’ DISTANCE DETERMINATION Globular clusters contain many RR Lyrae stars, which are a type of standard candle. These stars vary in brightness, and the period of variation relates to the star’s luminosity. Comparison of luminosity to apparent magnitude yields the distance. AGE DETERMINATION Astronomers plot the colors and magnitudes of cluster stars on an H-R diagram to get an oVerall picture of the eVolutionary states of the cluster stars. This, in turn, allows astronomers to constrain the age of the cluster. NOTABLE FEATURES/FACTS • M13 contains seVeral hundred thousand stars. Some estimates eVen go as high as one million. • The cluster is estimated to be 12 to 13 billion years old, making it one of the oldest structures in our Galaxy. • M13 contains at least two distinct populations of stars, indicating that the stars formed during multiple episodes. • Many “blue straggler” stars haVe been obserVed in M13. These are stars that appear much younger than the other stars in the cluster, largely due to their blue color. The outer layers of these stars may haVe been stripped away during interactions with other cluster stars, resulting in their blue color. • In 1974, the Arecibo radio telescope was used to send a signal to M13. The signal contained information including descriptions of DNA, human physiology, and our Solar System. If anyone is in M13 to receiVe it, we can expect a reply in about 50,000 years. • Messier added this object to his catalog on 1 June 1764. FALL SP TARGET INFORMATION MESSIER 6 (The Butterfly Cluster) BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Scorpius BEST VIEW: August DISCOVERY: GioVanni Batista Hodierna, c. 1654 DISTANCE: 1600 ly DIAMETER: 12 – 25 ly APPARENT MAGNITUDE: +4.2 APPARENT DIMENSIONS: 25’ – 54’ AGE: 50 – 100 million years DISTANCE DETERMINATION The colors and magnitudes of cluster members are plotted on an H-R diagram. Specific features, such as the location of the main sequence, haVe known theoretical positions on the plot. Comparison of the obserVed position of these features to the theoretical position can yield the distance. AGE DETERMINATION Astronomers plot the colors and magnitudes of cluster stars on an H-R diagram to get an oVerall picture of the eVolutionary states of the cluster stars. This, in turn, allows astronomers to constrain the age of the cluster. NOTABLE FEATURES/FACTS • There is some speculation that Ptolemy obserVed this cluster as early as the 2nd Century A.D. • The “Butterfly Cluster” nickname is attributed to Robert Burnham, Jr., who described the cluster as a “charming group whose arrangement suggests the outline of a butterfly with open wings.” This arrangement is best seen in binoculars. • M6 contains about 80 stars, but some estimate go as high as seVeral hundred. • The brightest star in M6 is an orange supergiant known as BM Scorpii. It is a semi-regular variable whose brightness varies oVer about 1.5 magnitudes. • Charles Messier cataloged this object on the night of 23 May 1764. FALL SP TARGET INFORMATION MESSIER 7 (Ptolemy’s Cluster) BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Scorpius BEST VIEW: August DISCOVERY: Claudius Ptolemy, 130 A.D. DISTANCE: 900 – 1000 ly DIAMETER: 20 – 25 ly APPARENT MAGNITUDE: +3.3 APPARENT DIMENSIONS: 80’ AGE: ~220 million years DISTANCE DETERMINATION The colors and magnitudes of cluster members are plotted on an H-R diagram. Specific features, such as the location of the main sequence, haVe known theoretical positions on the plot. Comparison of the obserVed position of these features to the theoretical position can yield the distance. AGE DETERMINATION Astronomers plot the colors and magnitudes of cluster stars on an H-R diagram to get an oVerall picture of the eVolutionary states of the cluster stars. This, in turn, allows astronomers to constrain the age of the cluster. NOTABLE FEATURES/FACTS • Claudius Ptolemy was first to catalog this cluster, listing it as a “nebula following the sting of Scorpius” in his Almagest. • M7 contains about 80 stars. • The brightest star in M7 is a type G8 yellow giant of magnitude 5.6. • Charles Messier cataloged this object on the night of 23 May 1764. FALL SP TARGET INFORMATION MESSIER 8 (THE LAGOON NEBULA) BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: August DISCOVERY: Hodierna, 1654 (nebula) Le Gentil, 1747 (nebula & cluster) DISTANCE: 4000 – 6000 ly DIAMETER: Approximately 130 ly x 60 ly APPARENT MAGNITUDE: +6.0 APPARENT DIMENSIONS: 90’x40’ DISTANCE DETERMINATION Astronomers can identify clusters of stars associated with the nebula, and then plot their colors and magnitudes on an H-R Diagram. Comparing the apparent magnitudes of cluster stars with known absolute magnitudes for giVen points on the diagram can yield the distance. This is very difficult because many stars haVe some leVel of obscuration by the nebula. Thus, the distance to M8 is uncertain. AGE DETERMINATION The presence of features associated with pre-main-sequence stars proVides an upper limit to the age. Astronomers can also plot the colors and magnitudes of nebula stars on an H-R Diagram to get an oVerall picture of the eVolutionary states of those stars and further constrain the age. NOTABLE FEATURES/FACTS • M8 is embedded in a large molecular cloud, a huge complex of cool gas that has the potential to produce thousands of stars. • The energy emitted by the young stars within M8 is responsible for the Visibility of the nebula. Starlight triggers emission in gaseous regions, giVing the nebula its distinctiVe glow. • Most obserVers will see the nebula as mostly gray in color, but some may pick out a green tint. This is due to OIII emission (doubly-ionized oxygen). • The open cluster NGC 6530 is part of the same molecular cloud as M8. The stars of the cluster appear slightly in front of the nebula from our perspectiVe. NGC 6530 contains seVeral hunded stars and is about two million years old. • SeVeral Bok Globules are present in M8. These are collapsing protostellar clouds that appear as dark spots within the nebula. • At the center of M8 lies the Hourglass Nebula. This is the brightest part of M8 and is an actiVe site of star formation. • The Hubble Space Telescope has detected seVeral large, funnel-shaped clouds associated with strong stellar winds in M8. The clouds are 0.5 ly long. • Messier added this object to his catalog on 23 May 1764. FALL SP TARGET INFORMATION MESSIER 20 (THE TRIFID NEBULA) BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: August DISCOVERY: Messier, 1764 DISTANCE: ~5200 ly DIAMETER: 42 ly APPARENT MAGNITUDE: +9.0 APPARENT DIMENSIONS: 11’ DISTANCE DETERMINATION Astronomers can identify clusters of stars associated with the nebula, and then plot their colors and magnitudes on an H-R Diagram. Comparing the apparent magnitudes of cluster stars with known absolute magnitudes for giVen points on the diagram can yield the distance. This is very difficult because many stars haVe some leVel of obscuration by the nebula. Thus, there is some discrepancy in quoted distances, but the most frequently adopted value is listed aboVe. AGE DETERMINATION The presence of features associated with pre-main-sequence stars proVides an upper limit to the age. Astronomers can also plot the colors and magnitudes of nebula stars on an H-R Diagram to get an oVerall picture of the eVolutionary states of those stars and further constrain the age. NOTABLE FEATURES/FACTS • The name “Trifid” refers to the three-lobed appearance of the nebula. The lobes are diVided by a dark nebula, known as Barnard 85. • A cluster of young stars is embedded in the central region of the nebula. These stars are estimated to be as young as 300,000 years old. • Most of the emission in the heart of the nebula is attributed to a single O- class star. • M20 contains both an emission and a reflection nebula. The central star has sufficient energy to excite atoms in the gas nearby, but it lacks the energy to prompt emission in the outskirts of the nebula. The outer portion of the nebula glows instead by reflected light. • Messier added this object to his catalog on the night of 5 June 1764. FALL SP TARGET INFORMATION MESSIER 17 (THE SWAN NEBULA) BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: Late August DISCOVERY: Philippe Loys deCheseaux, 1745 DISTANCE: 5200 – 6800 ly DIAMETER: 15 – 20 ly APPARENT MAGNITUDE: +6.0 APPARENT DIMENSIONS: 11’ DISTANCE DETERMINATION Astronomers can identify clusters of stars associated with the nebula, and then plot their colors and magnitudes on an H-R Diagram. Comparing the apparent magnitudes of cluster stars with known absolute magnitudes for giVen points on the diagram can yield the distance. ObserVations are made in infrared light because the nebula obscures the cluster stars. This heaVy obscuration makes the distance to M17 uncertain. AGE DETERMINATION Astronomers plot the colors and magnitudes of nebula stars on an H-R Diagram to get an oVerall picture of the eVolutionary states of the nebula stars and constrain the age. As with distance measurements, the dense nebular material makes these obserVations difficult and creates some uncertainty in the age value. NOTABLE FEATURES/FACTS • M17 is known by many names: the Swan Nebula, the Omega Nebula, the Checkmark Nebula, the Lobster Nebula, and the Horseshoe Nebula. • The brightest part of M17 is lit from within by about a dozen O-class stars. These stars are part of a larger cluster of up to 800 stars embedded in the heart of the nebula.
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