Star Hopping
This is an advanced lab activity which requires familiarity with the telescope, precise focusing, and careful observing.
Summary: Star hopping is a method used to locate objects in the night sky. To star hop, the observer uses well known or well charted bright objects to find his/her way to a target object. This is a good way to find objects in parts of the sky that are unfamiliar or to find faint objects.
Star hopping is the technique used to find countless faint objects in the sky such as star clusters, nebulae, galaxies, and planetary nebulae. Some of these are truly beautiful. Keep in mind that most of these are faint and are adversely affected by the light pollution from the city lights.
You would be quite surprised to see some of these objects from a dark sky....
Due Date: Decmeber 4.
Goal: In this lab, you will find as many different stars and celestial objects as you can. By doing so, you will quickly improve your skills with the telescope as well as become much more familiar with the stars and constellations in the sky. You also get to look at several types of objects discussed in class that are beyond the solar system. These are known as "deep sky" objects. This lab will probably take more than one lab period.
To complete this lab, you need to observe:
- 6-8 bright stars (list #1)
- 6 double stars (list #2)
- 4 deep sky objects (list #3)
- At least 2 stars known to have planets (list #4)
Where to Start: Choose objects that are visible on that particular night (consult your star charts), preferably those that are closer to the meridian. This is the line that goes from the North cardinal point, through the zenith (overhead) and to the South cardinal point. Avoid searching for objects anywhere close to the horizon, except for bright, naked-eye stars.
Procedure
Successful star hoping depends on your knowledge of constellation and your ability to use star charts and the finderscope. It requires patience and perseverance. It is essential to have the finderscope well aligned with the main telescope. You need star charts that show the your target and nearby stars. For some objects, such as bright stars, bright double stars, and a few "deep sky" objects located near bright stars (such as M57, M35, M44, M42) you can use the SC001 or SC003 charts. For the fainter objects, you need more detailed star charts that will be provided by the TAs (to be returned at the end of the lab). Make sure you get the chart appropriate for the objects you are looking for.
Check the alignment of the finderscope and start with the 25mm eyepiece.
Locate your target on the star chart and identify naked eye stars surrounding it (on the chart and in the sky).
Design a path on the star chart that connects one nearby (within 10o or so) naked eye star to you target. This will be your "star hop"
At this point, it is very useful to know the field of view of the finderscope. Using the declination scale on the star chart, to get a sense of scale for the chart, you will know the size of the field of view on the star chart, an additional aid in finding your way.
Center the naked eye star (your starting point) in the finderscope and follow your "star hop" through the finderscope to the target location, jumping from star to star, using the particular patterns of stars and star brightnesses that surround your target. Be careful at all times to identify distinctive patterns. While you may see more of fewer stars than are shown on the chart, the patterns you see through the finderscope and on the chart must be identical. Note that the target may not be visible in the finderscope. In that case set the cross hair to the target location using surrounding stars as reference points. REMEMBER: The view in the finderscope in inverted (upside down). Don't forget this or you will get lost in space! You can turn the chart upside down to match the view. Another hint: Stars appear brighter through the finderscope (and the main scope) than to the naked eye.
Have a look in the main telescope. Depending on how careful you have been, your object may or may not be in the field of view. Keep in mind what you are looking for.
A double star is usually easy to recognize if the components are far enough apart. You may have to switch to the 10mm eyepiece to be certain.
Nebulae can be VERY faint (and, if it is a bad night, not visible at all). Double and triple check your position before declaring that you can't see it.
Planetary nebulae can be very small in the 25mm eyepiece. You will need to inspect each stellar image carefully (focus well!) to tell the nebula apart from stars. You can verify that you found the right object by switching to the 10mm eyepiece.
Globular clusters are round, fuzzy aggregation of faint stars. Depending on the conditions, they will show well or be barely visible at all.
Open star clusters are concentrations of stars that usually stand out well from the background.
Galaxies are systems of hundreds of millions or billions of stars, "island universe" outside of and similar to our own Milky Way. Most galaxies are very difficult to see from the lab telescopes. They will show up as a fuzzy blob or a fuzzy patch, perhaps brighter towards the center.
Keep in mind that anything that doesn't look like a single star in the field is probably what you are looking for. Double check with your TA if in doubt.
If there's nothing in the field of view but background stars, then you missed. You need to scan the surrounding area in search of your target, keeping in mind that it may be just outside the field of view. Scan using the two slow-motion controls (RA & Dec), ONE AT A TIME. The idea is to scan E-W (2-3 fields of view), then move in declination by about 1/2 the field of view and repeat. Check regularly in the finderscope to ensure that you are still in the correct area and that you have not gotten lost.
If this fails, go back to your starting star and star hop again. You may have gotten confused in your star identifications. (Your instructor has been known to do this at times, so don't feel bad!)
Once you have found the object, make a few notes in your observing log about it, based on its type.
- Star: how bright does it appear? Can you discern any color? Do you notice anything else about it?
- Double Star: how easy is it to separate the components? How far apart are they (what fraction of the field of view; note which eyepiece you are using)? Can you detect any color contrast? Does one star appear brighter; if so, which one and how much?
- Star Cluster: How easy is it to see? How rich (dense) is it? Can you see any stars of particular colors? Does it fill the field of view (note the eyepiece)? How centrally concentrated vs. spread out is it? Sktech the cluster in your log book.
- Nebulae and Galaxies: How easy is it to see? What does it look like? Sketch the nebula or galaxy in your log book.
Understanding Star Charts
Star Charts are pages in an astronomical atlas much as state and city maps are pages in a road atlas. The primary objects in a star chart are the constellations and the stars contained within them. The constellations have boundaries as well as unique names, much like counties on a road map. In addition, a grid is normally superimposed on the constellations that shows the equatorial coordinate system (RA and Dec). The RA will be marked of in hours from 0 to 24 mostly along the bottom and top of the graph and the Dec will be marked off in degrees from -90 to +90 mostly along the left and right edges of the graph. Most of the objects placed on the chart will be stars of different sizes representing different brightnesses. Other objects such as nebulae and galaxies will have their own symbols. There might also be a few additional lines showing the apparent path of the sun over the year (the ecliptic) and the plane of the Milky Way galaxy.
A few of the objects on a star chart will have numbers and letters that serve as identifiers for those objects. The brightest stars will have Greek letters that represent Bayer catalog identifiers. They are placed such that an earlier letter represents a brighter star. For example, the second brightest star in the constellation Lyra is Beta Lyrae. Numbers next to a star are from the Flamsteed catalog. They represent an ordering of the stars within a constellation by increasing RA. Thus the bright star with the lowest numerical value of RA in the constellation Orion will be denoted 1 Orionis. The Flamsteed catalog includes fainter (and consequently more) stars, but there are still many more even fainter stars with neither identifier. Some of these will have other identifiers based on other catalogs.
Non-stellar objects have two major catalogs to represent them. The first is the Messier catalog, denoted by a capital M follow by a number. For example, the Great Nebula in Orion happens to be M42. This catalog has 110 entries, which are usually the brightest and more easily seen. A much more complete catalog is the New General Catalog with about 7800 entries. They are denoted by a capital NGC followed by a number. Some charts leave out the letters "NGC " to minimize clutter. So if you see and object that is not a star but has just a number next to it, it is almost always an NGC identifier. A particularly bright or interesting object may be included in many catalogs and thus have many identifiers, but, in general, the oldest identifier is the one that is most often displayed on the chart.
Star Hopping and Star Charts: Here are some more tips for star hopping. Hops should be done using the finder scope, since it provides a wider field of view and will thus give you more stars to compare with the star chart. Each "hop" should move you in the general direction of the final object and should be relatively short in angular size. Ideally, each hop should move a new recognizable pattern of stars into the edge or the center of the field of view (for instance, a distinctive pair or triangle of stars you identified on the star chart). After each hop, compare the star chart to what you see in the finder scope to make sure you know where you are. Be aware that the dimmest stars that are plotted on the star charts may not be visible with your finder scope under the light-polluted sky of the observing facility.
In some cases, your target will be due North (or East, etc) of a star, something you can see by inspecting the star chart. In that case, all you need to do is to center the star in the telescope (25mm eyepiece) and move the telescope in the required direction. Do this using the slow motion controls while looking through the eyepiece. You should see your target enter the field. If you go too far when trying this method, try using a standard star-hop with the finder scope.
Orientation of the field of view: Another way to orient yourself if there are not enough stars in the scope to do so, is to remember your coordinate system. If you lock the RA wheel and nudge the scope in Declination, you will be moving either North toward Polaris or South. The stars, since they are fixed, will of course move in the opposite direction. So a nudge in the direction of Polaris will make the stars appear to move toward the South. To determine East and West you could turn of the tracking motor and watch stars drifting toward the West. Then it becomes easy to compare the view from the scope with the star chart.
Using coordinates to find objects: This is an alternative method to star hopping that uses the coordinate dials of the telescope. Instead of going through the sometimes frustrating task of star hopping, you can use the known coordinates of celestial objects to locate them. The location of objects on the celestial sphere is analogous to that on the surface of the Earth. Geographical coordinates are two angles, the latitude and the longitude and uniquely specific a point on the surface of the Earth. In the sky, we use declination and right ascension. Like latitude, declination is the angle between the object and the celestial equator. It runs from -90o at the south celestial pole through 0o on the celestial equator and +90o at the north celestial pole. Right ascension is similar to longitude and is measure from a reference meridian in the sky. Instead of running from 0o to 360o, however, it runs from 0 to 24 hours, increasing toward the East. This makes sense since the celestial sphere appears to make one complete turn in 24 hours.
With these telescopes, it is best to use not the absolute coordinates of an object, but the difference in coordinates between the target and some nearby star (say, within 10o or so). Given your target use the star chart to identify a suitable nearby naked eye star. Acquire the star in the telescope and center it in the 25mm eyepiece. Calculate the difference in declination and right ascension between your target and your star, using star charts (or the table below) to get the coordinates. Using the dials, move the telescope by the required number of degrees in Dec and minutes in RA. Be careful to go in the right direction! This needs to be done with an accuracy of better than 1o since the field of view of the 25mm eyepiece is only about 0.6o. Make a 1o error and the object won't be in the field of view. In that case, start scanning the area (#6 above) and if you get impatient, start over again.
Object List #1: Bright Stars
Note: There's no need to "star hop" to stars this bright! These are starting points for star hopping! Look at 6 to 8 of these stars,choosing a variety of spectral types. In your observing notes, remember to comment on the color of each star and see if you can relate the color and the spectral types of stars.
| Name | Constellation | R.A. | Dec. | Vis. Mag. | When to View (Fall / Spring) | Distance | Sp.Type | Comment |
|---|---|---|---|---|---|---|---|---|
| Dubhe | Ursa Major | 11h 04m | +61d 45' | 1.8 | Early / Middle | 105 LY | K0 II | 34th brightest |
| Spica | Virgo | 13h 25m | -11d 09' | 1.0 | Early / Late | 275 LY | B1 V | 16th brightest |
| Arcturus | Bootes | 14h 16m | +19d 11' | 0.0 | Early / Late | 37 LY | K2 III | 4th brightest |
| Rasalhauge | Ophiuchus | 17h 35m | +12d 34' | 2.1 | Middle / --- | 60 LY | A5 III | 55th brightest |
| Vega | Lyra | 18h 37m | +38d 47' | 0.0 | Middle / --- | 27 LY | A0 V | 5th brightest |
| Altair | Aquila | 19h 51m | +08d 52' | 0.8 | Middle / --- | 16.5 LY | A7 IV | 12th brightest, 64th closest |
| Deneb | Cygnus | 20h 41m | 55d 17' | 1.3 | Middle / --- | 1600 LY | A2 I | 19th brightest |
| Alderamin | Cepheus | 21h 18m | +62d 35' | 2.4 | Middle / Early | 52 LY | A7 IV | 86th brightest |
| Fomalhaut | Pisces Austrinis | 22h 58m | -29d 37' | 1.2 | Middle / --- | 23 LY | A3 V | 18th brightest |
| Markab | Pegasus | 23h 05m | +15d 12' | 2.5 | Middle / Early | 110 LY | A0 III | 30th brightest |
| Alpheratz | Andromeda | 00h 08m | +29d 05' | 2.1 | Middle / Early | 120 LY | B9 IV | ... |
| Schedar | Cassiopeia | 00h 40m | +56d 32' | 2.2 | Middle / Early | 150 LY | K0 II | 65th brightest |
| Hamal | Aries | 02h 07m | +23d 27' | 2.0 | Middle / Early | 75 LY | K2 III | 48th brightest |
| Polaris | Ursa Minor | 02h 32m | +89d 15' | 2.0 | All / All | 300 LY | F7 I | 49th brightest; "The North Star" |
| Menkar | Cetus | 03h 02m | +04d 05' | 2.5 | Middle / Early | 150 LY | M2 III | 91th brightest |
| Mirfak | Perseus | 03h 24m | +49d 52' | 1.8 | Middle / All | 570 LY | F5 I | 33rd brightest |
| Capella | Auriga | 05h 17m | +46d 00' | 0.1 | Late / All | 45 LY | G5 III | 6th brightest |
| Aldebaran | Taurus | 04h 36m | +16d 31' | 0.9 | Late / All | 68 LY | K5 III | 13th brightest |
| Betelgeuse | Orion | 05h 55m | +07d 24' | 0.5 | Late / All | 520 LY | M2 I | 10th brightest |
| Sirius | Canis Major | 06h 45m | -16d 43' | -1.5 | Late | 8.6 LY | A1 V | 1st brightest, 8th closest |
| Castor | Gemini | 07h 35m | +31d 54' | 1.6 | --- / Middle | 49 LY | A2 V | 23rd brightest. double star, 3" sep. |
| Procyon | Canis Minor | 07h 39m | +5d 14' | 0.4 | --- / Middle | 11 LY | F5 V | 8th brightest |
| Regulus | Leo | 10h 08m | +11d 59' | 1.5 | --- / Late | ... | ... | ... |
Object List #2: Double Stars
Notes: Position Angle refers to the location of the secondary (the fainter of the two stars) with respect to the primary (the brighter member of the double). Astronomers measure angles counterclockwise from north (the 12 o'clock position). Separation refers to the angular distance between the pair, in units of seconds of arc. Magnitude is the apparent magnitude of the two members of the binary, fainter stars have larger magnitudes. Take notes about the appearance of the double star: relative brightness of the components, separation, and color contrast if any.
| Name of Primary | Constellation | RA | Dec | Position Angle | Separation | Visual Mag. | Comment |
|---|---|---|---|---|---|---|---|
| Beta | Cepheus | 21h 29m | +70 34' | 249 | 13.3" | 3.2, 7.9 | ... |
| Alpha | Cassiopeia | 00h 41m | +56d 32' | 282 | 69.5" | 2.2, 8.9 | ... |
| Eta | Cassiopeia | 00h 49m | +57d 49' | 293 | 12.2" | 3.4, 7.5 | ... |
| Alpha | Canes Venatici | 12h 56m | +38d 19' | 228 | 19.3" | 2.9, 5.5 | ... |
| Epsilon | Perseus | 03h 58m | +40d 01' | 10 | 8.8" | 2.9, 8.1 | ... |
| Eta | Perseus | 02h 51m | 55d 54' | 300 | 28.3" | 3.3, 8.5 | ... |
| Gamma | Aries | 1h 53m | +19d 17' | 0 | 7.8" | 4.5, 4.5 | ... |
| Alpha | Hercules | 17h 15m | +14d 23' | 110 | 4.6" | 3-4, 5.4 | ... |
| Gamma | Leo | 10h 20m | +19d 51' | 125 | 4.6" | 2.6, 3.5 | ... |
| Alpha | Ursa Minor | 02h 32m | +89d 16' | 218 | 18.4" | 2.0, 9.0 | Alpha Ursa Minoris is, of course, Polaris |
| Theta 1 | Orion | 05h 35m | -5d 24' | --- | 13"-16" | 5.4, 6.3, 6.7, 6.8 | Quadruple system known as "the Trapezium," at center of Orion Nebula. |
| Zeta | Ursa Major | 13h 24m | +54 56' | 152 | 14.4" | 2.3, 4.0 | Zeta Ursa Majoris is a wide double with the brighter separated into two, i.e. a triple system. It is the closer pair that is given here. |
| Gamma | Andromeda | 02h 04m | +42d 20' | 063 | 9.8" | 2.3, 4.8 | ... |
| Beta | Cygnus | 19h 31m | +27d 58' | 054 | 34.4" | 3.1, 5.1 | Notice the nice color constrast between the two stars in both Gamma Andromeda and Beta Cygnus. Why? |
| Epsilon | Lyra | 18h 44m | +39d 40' | +173 | 207.7" | 4.7, 5.1 | Epsilon Lyrae' stars are both doubles themselves (a double double). The separation is very small (2.5 arc seconds) and is best viewed in the 10mm eyepiece. |
| Zeta | Lyra | 18h 49m | +37d 36' | 150 | 43.7" | 4.3, 5.9 | ... |
| Nu | Draco | 17h 32m | +55d 11' | 312 | 61.9" | 4.9, 4.9 | Notice the lack of color contrast between the two stars in both Zeta Lyra and Nu Draco. Why? |
| 61 | Cygnus | 21h 07m | +38d 44' | 144 | 31" | 5.3, 5.9 | Famous nearby double, 11.1LY away. 4th nearest naked eye star, first star with measured parallax (by Bessel in 1838). |
| Omicron 2 | Eridanus | ... | ... | 105 & 347 | 89" & 8" | 4.5, 9.5, 11 | Unusual Triple system. Primary is a K1 V star; secondary is a white dwarf; third and faintest companion is a low-mass M5 V star. |
| Iota | Cassiopeia | 2h 29m | +67d 24' | 241 & 114 | 2.2" & 7.3" | 4, 7, 8 | Triple system. |
Object list #3: Clusters, Nebulae, and Galaxies
Notes: Most of these objects are challenging when seen from a large city. You may experience difficulty seeing them even if the telescope is pointing in the right spot! They are nevertheless among the most beautiful objects in the night sky and quite interesting to look at. As an aside on light pollution, you should know that while faint, several of these objects are visible to the naked eye under a very dark site. Always start with the 25 mm eyepiece to locate the object. For the smaller ones (planetary nebulae), you may have to switch to the 10mm to clearly distinguish the object from the surrounding stars. In all cases, use both magnifications (eyepieces) when making your observations as one may reveal something the other doesn't. If you have difficulty finding these objects with the star hopping method, you may have more luck using coordinate offsets from a nearby star you can locate easily (such as in the "Bright stars" list above).
Sketch the object in your logbook. Indicate the orientation and scale of your drawing and add notes to complement your sketch.
*** = extra challenging. Good luck!
| Name | Constellation | RA | Dec | Size (arcmin) | Distance | Comments |
|---|---|---|---|---|---|---|
| M42: Orion Nebula | Orion | 5h 35m | -5d 23' | 10' | 1600 LY | Region of Star formation. Surrounds multiple star Theta 1 Ori. |
| M31: Andromeda Galaxy | Andromeda | 0h 43m | 41d 16' | 190' | 2.6 MLY | Spiral Galaxy. Also look for M32, a small elliptical galaxy, 24" to the north (both are visible in one 25mm field of view). |
| M34 | Perseus | 2h 42m | +42d 46' | 30' | 1500 LY | Bright galactic cluster. Age = 100 Myr. |
| NGC884/869 | Perseus | 2h 19m | +57d 08' | 29' / 29' | 7400 LY | Famous Double cluster. Age = 11Myr / 6Myr. |
| M45: The Pleiades | Taurus | 3h 47m | +24d 07' | 100' | 400 LY | Very bright, very large galactic cluster. Age = 70 Myr. You can see the brightest stars naked eye. With a largeish telescope under a dark sky, you can see faint nebulosity around the stars. |
| M41 | Canis Major | 6h 46m | -20 45' | 38' | 2400 LY | Galactic cluster 4d south of Sirius. 100 Myr old. |
| M44: The Beehive | Cancer | 8h 40m | +19d 40' | 80' | 520 LY | Very large galactic cluster. 660 Myr old. |
| *** M13 | Hercules | 16h 42m | +36d 28' | 10' | 22000 LY | Globular cluster framed by 2 stars of 6th mag |
| *** M92 | Hercules | 17h 17m | +43d 08' | 8' | 25000 LY | Globular cluster, compare with M13 |
| M11 | Scutum | 18h 51m | -6d 16' | 13' | 5600 LY | Very dense galactic cluster. 224 Myr old. |
| M57 | Lyra | 18h 54m | +33d 02' | 70'' | 2300 LY | Famous Planetary Nebula ("Ring nebula"). Neb is 3900 years old |
| NGC6826 | Cygnus | 19h 45m | +50d 32' | 25'' | 2300 LY | "Blinking " planetary nebula. Look for central star! Neb. is 5200 years old |
| *** M15 | Pegasus | 21h 30m | +12d 10' | 5' | 32000 LY | Globular cluster. |
| *** M36 | Auriga | 5h 36m | +34d 08' | 12' | 4100 LY | 25 Myr old galactic cluster |
| *** M35 | Gemini | 6h 09m | +24d 21' | 25' | 2200 LY | 107Myr old galactic cluster. |
Object List #4: Stars Known to Have Planetary Companions
Notes: This is a list of several solar-type stars known to have planets in orbit around them. (Many more planets than the ones on this list are known to exist!) The planets themselves are hopelessly beyond the reach of amateur telescopes (in fact, they have not yet been seen directly, even with the most advanced telescopes used by astronomers), but it is interesting to see what a star like the Sun looks like from afar and think that it has planetary companions, some perhaps similar to those in our solar system. These stars are not visible to the naked eye from campus, but can be glimpsed in the finderscope. They are an easy sight through the telescope. What would the Sun and its family of planets look like to an alien civilization living on a planet orbiting such a star? How hard would it be to identify the stars worth investigating for signs of (perhaps intelligent) life? Take time to make notes and record your impressions.
| Name | RA | Dec | Sp. Type | Distance | Visual Mag. | Planet Mass (Jupiter = 1) | Orbital Pd. | Comment |
|---|---|---|---|---|---|---|---|---|
| 51 Pegasus | 22h 57m | +20d 46' | G2 V | 44 LY | 5.5 | >0.5 | 4.25 days | ... |
| 47 Ursa Major | 10h 59m | +40d 26' | G0 V | 45 LY | 5.1 | >2.4 | 3.0 yr | ... |
| Upsilon Andromeda | 1h 37m | +41d 24' | F8 V | 54 LY | 4.1 | >0.6 | 4.6 days | System of 3 planets! |
| Rho 1 Cancri | 8h 52m | +28d 20' | G8 V | 44 LY | 6.0 | >0.8 | 14.8 days | Northernmost of a pair |
| Tau Bootes | 13h 47m | +17d 28' | F7 V | 62 LY | 4.5 | >3.9 | 3.3 days | ... |
| 16B Cygnus | 19h 42m | +50d 31' | G2 V | 96 LY | 6.0 | >1.6 | 2.2 yr | Wide double star. B is to the SE (PA=135) |
| Rho Corona Borealis | 16h 01m | +33d 19' | G1 V | 55 LY | 5.4 | >1.1 | 40 days | ... |