The Elephant Trunk Nebula (IC 1396) By Bob Holder

Transcription

1 Bristol Astronomical Society Information Leaflet August 2015 The Elephant Trunk Nebula (IC 1396) By Bob Holder Solar Eclipse, Castle Park, Bristol, 20 th March

2 Observing Calendar August 2015 Observing Calendar August

3 The Sun The Moon Sun s Position at Midday 15th August 2015 August 7th and 23rd: The Straight Wall Solar Events for 2015 The Straight Wall (Rupes Recta) in Mare Nubium is best observed either 1 or 2 days after First Quarter (23rd August: evening) or a day or so before Third Quarter (evening of the 7th August). To be honest, it is not really a wall but a gentle scarp - as Sir Patrick Moore said: "Neither is it a wall nor is it straight!". It is 68 miles (110 km) long, approximately 1.5 miles (2.5 km) wide, and about 800 to 1,000 feet ( m) high. See page 6 for location and some lunar images

4 The Moon The Planets August 1st at 00:00 August 15th at 00:00 August 31st at 00:00 The Straight Wall at Sunrise and Sunset

5 The Planets Mercury This month Mercury returns to the twilight sky before sunset increasing in brightness from +1.2 to +0.1 during the month as it rises a little higher in the western sky. August 7th after sunset: Jupiter and Mercury close to Regulus in Leo: Given clear skies and a low western horizon it should be possible to spot Jupiter and Mercury within a one degree diameter circle with the star Regulus in Leo. Both planets will be then nearly fully lit, but Jupiter's 31 arc second disk will appear significantly less bright than Mercury's disk, some six times smaller. Venus Venus, rises at sunrise on the 18th of August, but 55 minutes before the Sun a week later and about one hour 30 minutes before the Sun by month's end. As it does so, it brightens from -4.1 to -4.5 magnitude and its angular size reduces slightly from 58 to 52 arc seconds, whilst the percentage illuminated area (phase) of the planet increases from 1 to 9%. So close to the Sun, it is not safe to observe after sunrise. Mars As August begins, Mars, shining at magnitude +1.7, rises about 70 minutes before the Sun. This increases to about 2 hours before sunrise by month's end with its brightness virtually unchanged at +1.8 magnitude. On August 8th, at an elevation of just 5 degrees, it will lie below Castor and Pollux in Gemini around 45 minutes before sunrise whilst on the mornings of the 20th and 21st it will lie close to M44, the Beehive Cluster in Cancer. The Planets Jupiter Jupiter passes behind the Sun on August 26th so can only be observed during twilight at the very beginning of the month low in the west after sunset. Venus passed 6 degrees below Jupiter on July 31st so will be close for the first few days of the month but becoming progressively more difficult to observe. Saturn Saturn is the only bright planet visible outside twilight this month. It lies in Libra near the wide double star Alpha Librae falling in brightness a little from +0.4 to +0.5 magnitudes during the month. It ceases its retrograde motion westwards in the sky on August 2nd and so begins to move eastwards back towards Scorpius. One hour after sunset it will lie just 20 degrees above the horizon so the atmosphere will limit the view of its 17 arc second disk but the ring system, now 24 degrees open, should still show nicely along with Titan, its largest satellite. Saturn will be 90 degrees east of the Sun (eastern quadrature) on August 21st so the globe's shadow on the rings is at its maximum extent giving a three-dimensional feel to our view of this, most beautiful, planet. August 22nd -Saturn close to a first quarter Moon and a lunar occultation of Theta Librae: Looking west one hour after sunset, Saturn, in Libra, will be seen close to a first quarter Moon. A good horizon in the South South-west will be needed. The star Theta Librae will lie very close to the Moon's dark limb and it should be seen to disappear behind the limb around 21:41 BST. 8 9

6 The Planets The Sky This Month Neptune August is a good month to observe Neptune with a small telescope. Neptune comes into opposition - when it is nearest the Earth - on the 31st of August, so will be seen well both this month and next. Its magnitude is +7.9 so Neptune, with a disk just 3.7 arc seconds across, is easily spotted in binoculars lying in the constellation Aquarius as shown on the chart below. It rises to an elevation of ~27 degrees when due south. Given a telescope of 8 inches or greater aperture and a dark transparent night it should even be possible to spot its moon Triton. Neptune lies very close to the 7th magnitude star HIP and, on the 5th of September, will lie directly above it. Aquarius & Neptune on 15th August at 00:00 Planetary info source: & 10 This map shows the constellations seen towards the south at about 10pm BST in mid August. High over head towards the north (not shown on the chart) lies Ursa Major. As one moves southwards one first crosses the constellation Hercules with its magnificent globular cluster, M13, and then across the large but not prominent constellation Ophiucus until, low above the southern horizon lie Sagittarius and Scorpio. To the right of Hercules lie the arc of stars making up Corona Borealis and then Bootes with its bright star Arcturus. Rising in the east is the beautiful region of the Milky Way containing both Cygnus and Lyra. Below is the constellation of Aquilla, the Eagle. The three bright stars Deneb (in Cygnus), Vega (in Lyra) and Altair (in Aquila) make up the "Summer Triangle". 11 Sky This Month :

7 The Sky This Month - Cassiopeia The Sky This Month - Cassiopeia Cassiopeia is famous for its distinctive W shape, an asterism formed by five bright stars in the constellation. The stars, from left to right, are Epsilon, Delta, Gamma, Alpha and Beta Cassiopeiae. Gamma Cassiopeiae is the central star in the W shape and the brightest star in the constellation. It is a blue star about 610 light years distant, with a luminosity 40,000 times that of the Sun, and having about 15 solar masses. It is an eruptive variable star that serves as a prototype of a class of variable stars. It exhibits irregular variations in brightness, which range between 2.20 and 3.40 magnitudes. The star rotates very rapidly and bulges outward along the equator. As a result of its fast spinning, a decretion disk of lost mass and material forms around the star, which causes the fluctuations in luminosity. It is also a spectroscopic binary star, an optical double with a magnitude 11 companion about two arc seconds away and an orbital period of circa 204 days. Beta Cassiopeiae (Caph) is a Delta Scuti type variable star see page 24. Cassiopeia contains several notable deep sky objects, among them the open clusters Messier 52 and Messier 103, and the star-forming cloud popularly known as the Pacman Nebula (NGC 281). M103 (NGC 581) M52 (NGC 7654) Messier 52 is an open cluster, approximately 5,000 light years distant. It has an apparent magnitude of 5.0 and can be seen with binoculars. The age of the cluster is estimated to be around 35 million years. It is 13 arc minutes (19 light years) in diameter. Messier 103 is another open cluster in Cassiopeia and has an apparent magnitude of 7.4. It lies approximately 10,000 light years from Earth and contains 172 stars. The age of the cluster is estimated to be about 25 million years. NGC 281 is a large gas cloud in which star formation has relatively recently taken place. It contains a large amount of ionised atomic hydrogen which is lit by the ultraviolet light of young, hot, blue stars. It is 9,500 light years from Earth and has an apparent magnitude of

8 Observing Notes Planetary Positions 15 August 1 August 23:00 15 August 22:00 31 August 21:

9 The Sky This Month - Cepheus The Sky This Month - Cepheus Cepheus is home to the Garnet Star (the famous red supergiant) and several well-known deep sky objects. Cepheus has one star with known planets (Gamma Cephei) and contains no Messier objects. The brightest star in the constellation is Alderamin (Alpha Cephei). Cepheus was named after the mythical King Cepheus of Aethiopia, husband of Cassiopeia and father of Andromeda, both represented by neighbouring constellations. Alpha Cephei (Alderamin): a white class A star, currently evolving from a main sequence star into a subgiant. It is approximately 49 light years distant. Its traditional name, Alderamin, is derived from an Arabic phrase meaning the right arm. The star has an apparent magnitude of 2.5. It rotates at a very high speed, at least 246 km/s, and completes one revolution within 12 hours or less. It can be seen by the naked eye. NGC 188 (Cald. 1) Garnet Star μ Cephei (Mu Cephei) : a red supergiant, approximately 1,870 light years distant. Mu Cephei is one of the most luminous stars known and is a class M bright supergiant, being one of the largest stars ever observed in the entire galaxy. It has a radius 1,650 times that of the Sun, or 7.7 Astronomical Units. The star s apparent magnitude varies between magnitude 3.62 and 5 in a period of 2 to 2.5 years, without a recognizable pattern. See page 25 for more details. The Iris Nebula (NGC 7023, Caldwell 4): a reflection nebula with an apparent magnitude of 6.8. It is approximately 1,300 light years distant. The object is really a star cluster embedded inside a nebula. The Iris Nebula is lit by the star SAO and it lies close to Beta Cephei, which has an apparent magnitude of NGC 188 (Caldwell 1): an open star cluster, approximately 5,400 light years distant and apparent magnitude 8.1. It was discovered by the English astronomer John Herschel in It is one of the oldest known open clusters. It lies about five degrees away from the North Celestial Pole NGC 7023 (Cald. 4) 17

10 The Sky Looking Overhead at Midnight mid-august 2015 The Sky Looking North at Midnight mid-august

11 The Sky Looking East at Midnight mid-august 2015 The Sky Looking West at Midnight mid-august

12 The Sky Looking South at Midnight mid-august 2015 Messier of the Month M15 Globular Cluster M15 (NGC 7078), class IV, in Pegasus Discovered by Jean-Dominique Maraldi in 1746, globular cluster Messier 15 is at a distance of about 33,600 light years. Its diameter of 18.0 arc mins corresponds to a linear extension of about 175 light-years, and its total visual brightness of +6.2 corresponds to an absolute magnitude of -9.17, or roughly 360,000 times that of our sun. Its overall spectral type has been determined as F3 or F4. The globular cluster is approaching us at 107 km/sec. M15 is perhaps the densest of all (globular) star clusters in our Milky Way galaxy the HST has photographically resolved its super-dense core. M15's core has undergone a process of contraction called "core collapse", which is common in the dynamical evolution of globular clusters. It is still unclear if the central core of M15 is packed so dense simply because of the mutual gravitational interaction of the stars it is made of, or if it houses a dense, supermassive object, which would be resembling the supermassive objects in galactic nuclei. M15 can be found extremely easily: Find the 2nd mag star Epsilon Pegasi, and Theta Pegasi SE of it. Follow the line from Theta over Epsilon and find M15 3 1/2 deg W and 2 1/4 deg N of Epsilon. With its apparent visual brightness of magnitude 6.2, M15 is about at the limit of visibility for the naked eye under very good conditions. Binoculars, reveal M15 as a round nebulous object, while it appears as a round mottled nebula in 4-inch telescopes,

13 Variable Star of the Month - Caph Caph β Cassiopeiae (Beta Cassiopeiae) Caph is unusual in being a " Delta Scuti" type of variable star, in fact the brightest of the type. Slightly unstable, and rapidly pulsating, a relatively low -mass version of the famed "Cepheid variables," the star varies by about 6 percent in brightness over a 2.5 hour period. Delta Scuti variables show fluctuations in luminosity due to both radial and non-radial pulsations on their surface. They are usually giants or main sequence stars of spectral types ranging between A0 and F5. Caph is a class F (F2) giant with a 6700 Kelvin surface, about 900 Kelvin warmer than the Sun. From its very close distance of only 54 light years, Caph is seen to shine with 28 times the brightness of the Sun. It is not unusual in having a small stellar companion, about which little is known, that orbits it every 27 days. Though called a "giant," the star is really not all that large, only about four times as big as the Sun. The term really refers to the state of Caph's evolution, hydrogen fusion in its core having recently stopped. The star is not so much a physical giant but becoming one, the core now contracting, the star cooling and expanding (though no such changes could be seen in a human lifetime). Star of the Month - Mu Cephei Mu Cephei - Herschel's Garnet Star (RA 21h 43m 31s Dec +58d 46m 48s) When Sir William Herschel observed Mu Cephei in 1783 he described it as a most beautiful object of a very fine deep garnet colour that is exceptionally striking when compared to nearby white stars. In fact it is an extremely luminous red supergiant and one of the reddest of all known stars. The star may even be the largest star visible to the naked eye with an estimated radius of 1.15 billion kilometres (710 million miles) or 1650 times that of the Sun. Mu Cephei is a red supergiant star of type M2 Ia. As with other red supergiants measuring accurately the distance to Mu Cephei is difficult. In 2005, a maximum likelihood estimate of the distance using a kinematics study by Famaey et al gave a value of 1870 ± 323 light-years (573 ± 99 parsecs). Assuming a distance of 1870 light-years, Mu Cephei is so large that if it were at the centre of our solar system it would reach somewhere between the orbits of Jupiter and Saturn. Currently, there are believed to be only a few known stars that are larger than Mu Cephei. In astronomical terms, Mu Cephei does not have long to live. The star has almost certainly stopped internal hydrogen fusion (likely fusing helium in its core into carbon) and will almost certainly destroy itself in a supernova. Caph is in a region of temperature and luminosity called the "Hertzsprung Gap" (after the famous Danish astronomer Ejnar Hertzsprung) in which we find few other stars. It will spend only about one percent of its few billion year lifetime in such a condition. Caph has a particularly faint "corona," a halo of hot, magnetically heated gas that radiates X- rays. Caph has a mean apparent magnitude of 2.28 (range 2.25 to 2.31) and its position is RA 00h 09m 11s, Dec ' 59". See page 13 for the constellation map and location of Caph

14 Perseid meteor shower Thu 13th Aug. The Perseid meteor shower will reach its maximum rate of activity on 13 August Some shooting stars associated with the shower are expected to be visible each night from 23 Jul to 20 Aug. The maximum rate of meteors expected to be visible from a dark location is around 80 per hour (ZHR). The Moon will be 28 days old at the time of peak activity, and so will present minimal interference. The radiant of the Perseid meteor shower is at around right ascension 02h20m, declination +58, as shown by the grey cross on the star chart below. At midnight, it appears 40 above our north-eastern horizon in Bristol. All of the meteors will appear to be travelling directly outward from this point. The best place to look to see as many meteors as possible is not at the radiant itself, but at any dark patch of sky which is around 90 away from it, since it is at a distance of around 90 from the radiant that meteors will typically appear at their brightest. BAS Open Observing Saturday Observing at the Failand Observatory Members open the Society's Observatory at Failand for the General Public on many clear Saturday Nights. We welcome visitors including family, friends, neighbours with or without telescopes and binoculars etc. No astronomical knowledge or skill is required except interest in what you may be about to observe! For further information on how to attend, including details and information on where to find us please saturday.observing@bristolastrosoc.org.uk. Use the status message on to check if the session is running. Date BAS Members 1 Aug Jane Clark Nigel Kirkland Stephen Price 8 Aug No public observing 15 Aug No public observing 22 Aug John Willis Alison Camacho Nigel Kirkland 29 Aug No public observing Observing at Tyntesfield & Other Events The Bristol Astronomical Society provides equipment and expertise for Star Parties and Solar Observing run by The National Trust, Tyntesfield. These take place (weather permitting) regularly during the year. Bristol Astronomical Society also regularly organises other star gazing events around the City. Check for more details and how to attend

15 Society News Programme of Events (At Bristol Grammar School, University Road BS8 1SR) 7 Aug Members Talk Night 21 Aug Summer Party Evening As normal over the summer period, from 12th June to 25th September 2015, meetings will be held once a fortnight and will be talks given by BAS members. Please check the Google calendar before each meeting for more details on the night. BAS Visit to the Norman Lockyer Observatory BAS members will be visiting the Norman Lockyer Observatory in Sidmouth, Devon, on 4th October The Norman Lockyer Observatory is both a historical observatory and home to an active amateur astronomical society. The observatory provides a planetarium and modern telescopes with computer enhanced imaging, as well as maintaining some of the nation's most historic instruments. There are five domes: McClean (12" refractor), Kensington (10" refractor), Victoria (12" reflector), Connaught (20" reflector), and the Mond dome which houses the famous 6¼-inch Lockyer refractor telescope. If you would like to join the BAS visit on the 4th October, please contact any of the BAS Committee members or the BAS Secretary. Bristol Astronomical Society Registered Charity No