Catch a Star 2002

Astronomical Observatory and Planetarium, Varna, Bulgaria

Contents:
1. Introduction
2. Discovery
3. Historical information: the Comet in the past
4. The Comet nowadays...
5. The Comet in the future...
6. In the present-orbital elements
7.The Nature of the Comet - A small physical and
chemical portrait of the comet Hale-Bopp
8. Comparisons of orbital elements for
C/1995 O1 Hale-Bopp, the Great comet of 1811
and C/1996 B2 Hyakutake
9. How did scientist receive the information
10. We make a picture of the comet
11. Exercise: Determination of the size of the coma and the tail using visual observations of the comet Hale-Bopp
12. Conclusion

1. Introduction
The Comet Hale-Bopp is one of the most impressive bright comets that have passed for the last 10 years. The appearance of such a comet with a beautiful long tail allows astronomers to make new observations and researches and it gives a pleasure people to observe a wonderful space show.

2. Discovery
The comet was discovered independently by two amateur comet observers: Alan Hale from New Mexico, and Thomas Bopp from Arizona, on July 23, 1995. At discovery, the comet was near magnitude 10.5 - which is not unusual for comets discovered typically within 1-2 AU from Earth. However, the slow motion of the comet against the background stars suggested that this comet was very far away, making the brightness very unusual. It was the brightest modern comet discovered by amateurs, appearing nearly 50,000 times brighter than comet P/Halley did as it approached the sun at the same distance.
Both observers were waiting for other objects to rise and were passing time by looking at some deep sky objects (globular clusters of stars) in the constellation of Sagittarius. It was immediately apparent that there was a fuzzy object in the field that was not on the usual star charts, and after confirming that the object was moving against the background stars, both observers reported the discovery to the Central Bureau for Astronomical Telegrams in Cambridge, MA. Once it was confirmed that this was a new comet, it was given the designation C/1995 O1 (Hale-Bopp).

3. Historical information
In the past:
There are no historical information about this comet because its previuos passage near the Sun and the Earth was 4 200 years ago. During that time the comet was supposed to have been observed by priests from the oldest observatory in New Greng in Ireland. In the past the appearance of a comet on the night sky caused fear while today comets are wonderful sky attractions for people and they are very interesting objects for examinations.

4. Nowadays:
Everyone of us dreams to find a new comet. We know that it is needed a lot of time a comet to be found. The short stories of both comet observers
show us that comet observers need not only tenacity but
and luck.
Alan Hale's story: "I love the irony -- I've spent over 400
hours of my life looking for comets, and haven't found
anything, and now, suddenly, when I'm not looking for
one, I get one dumped in my lap. I had obtained an
observation of P/Clark earlier, and needed to wait an
hour or so before P/d'Arrest got high enough to look at,
and was just passing the time til then, and I decided to
look at some deep-sky objects in Sagittarius. When I
turned to M70, I saw a fuzzy object in the same field, and
almost immediately suspected a comet, since I had been
looking at M70 last month, and *knew* there wasn't any
other objects there."
Thomas Bopp's Story : "On the night of July 22, 1995
some friends and I headed out into the desert for a dark of
the moon observing session. The site, which is west of
Stanfield, AZ and a few mile south of Interstate 8 is about
90 miles southwest from my home.
My friend Jim Stevens had brought his 17-1/2" Dobsonian.
We started the evening observing some of the Messier
objects such as the Veil and North American Nebulae in
Cygnus, when Jim said " Let's look at some of the globulars
in Sagittarius." We started our tour with M22 and M28,
observing at 50X and then at 180X.
Around 11:00 local time, we had M-70 in the field when Jim went to the charts to determine the next object of investigation. I continued watching M-70 slowly drift across the field, when it reached a point 3/4 of the way across a slight glow appeared on the eastern edge. I repositioned the scope to center on the new object but was
unable to resolve it. I called to Jim and asked him if he knew what it might be, after a visual inspection he stated he wasn't familiar with it but would check the charts. After determining the general position of the object he was unable to find it on either Sky Atlas 2000.0 or Uranometria.
The moment Jim said "we might have something" excitement began to grow among our group and I breathed a silent prayer thanking God for his wondrous creation. My friend, Kevin Gill then took a position from his digital setting circles and estimated a magnitude.
At 11:15 I said that we needed to check the object for motion and should watch it for an hour. The group observed it change position against the star field over that period and at 12:25 I decided to drive home and report our finding.
Arriving at home initial attempts to send the telegram were unsuccessful due to an incomplete address I had. After searching my library I was able to located the correct address and confirmation was requested.
At 8:25 AM July 23, 1995 Daniel Green of the Harvard Smithsonian Astrophysical Observatory telephoned and said, "Congratulations Tom, I believe you discovered a new comet." and that was one of the most exciting moments of my life."
In the observing history the comet Hale-Bopp had very important dates and distances.
When the comet was found- on 23.07.1995- it was at a distance of 7.14 AU from the Sun. Its nearest passage to the Earth was on 22.03.1007 at a distance of 1.315AU. It was nearest to the Sun on 1.04.1997 when it was at a distance of 0.914 AU.
It is very interesting that the comet had been photographed 2 years earlier. On August 2, 1995, shortly after its discovery pre-discovery images were found on photographic plates taken at the Anglo-Australian Observatory on 4/27/93, when the comet was 13.1 AU from the sun. The image showed that the comet was active (i.e. had a coma and tail) even at this distance. Because of the long time baseline of the observations, it was possible to quickly determine an orbit. It was found that the last perihelion passage of the comet was nearly 4200 years ago.

5. In the future:
In the absence of non - gravitational forces, the comet should next return after approximately 2 380 years. It is possible the comet to be again so bright and beautiful and with a long tail. After 2 380 year the comet will go towards the Sun again for 3rd time and that's why it will preserve a huge part of its mass and the comet will be very active. The comet won't change much but for such a long period sciences and people on the Earth will have changed so much that it is absolutely possible then a space project "Catch a comet" to exist.

6. In the present-orbital elements
From 3000 observations made during the period 1993 Apr. 27-2001 Feb. 17 S.Nakano Minor Planet calculated the following very harp orbital elements of the comet:
C/1995 O1 (Hale-Bopp)
Epoch 1997 Mar. 13.0 TT = JDT 2450520.5
Perihelion passage time : T 1997 Apr. 1.1373 TT
Perihelion passage distance (AU) : q = 0.914142
Argument of perihelion (deg.) : Peri. = 130.5887
Longitude of the ascending node (deg.): Node = 282.4707
Eccentricity : e = 0.995068
Inclination (deg.): Incl. = 89.4300
We decided to look for orbital elements of other 2 comets and to compare them with the orbital element of the comet Hale-Bopp

Comparisons of orbital elements
for C/1995 O1 Hale-Bop, the Great comet of 1811and C/1996 B2 Hyakutake

From the table it can be seen that the comet Hale-Bopp is similar to the comet from 1881's because both comets have similar periods of passage, they have one and the same distances to the earth and have the same eccentricity. The comet C/1995 O1 Hale-Bopp and the comet C/1996 B2 were very bright and could be seen with naked eyes. Both comets had very long tails and huge comas. Both comets have similar Argument of Perihelion, and this means that they had the same orientation of the orbit in its own plane but these three comets had absolutely different Ascending Node.

7. The Nature of Comet - A small physical and chemical portrait of the comet Hale-Bopp
Brightness - The brightness of comet Hale-Bopp has been exceptional, and the predictions indicate that it could rival the Great Comet of 1811. Comet Hale-Bopp reached this brightness at a distance of 3 AU from the sun - the visible magnitude off the comet reached -0m.8.
How fast a comet will brighten as it approaches perihelion will depend on the type of orbit it has. Comets, which have been close to the Sun before tend to brighten much more quickly than those comets, which are making their first passage, however, in both cases there can be unexpected brightness outbursts, which can make predicting the increase difficult. Astronomers have also never observed a comet in this type of orbit so far before its perihelion, which makes predicting the brightness more difficult. Typically a comet's visual brightness may be represented by a power-law formula:

visual magnitude = absolute magnitude + 5 log (delta) + 2.5 n log (r)

where the absolute mag is related to the size of the nucleus, delta is the geocentric distance, and r is the heliocentric distance. The value of n determines how fast the comet is brightening. The early observations suggested n=4, then the brightness leveled off, and now the comet is brightening with n=3, however, the comet should not be a disappointment.

How "big" are the comet's coma and nucleus :
The coma or the head of a comet, the dust and gases that surrounds the nucleus, can reach several 100,000 kilometers in diameter. This size varies with the comet's distance from the Sun. The tail of a comet, like Hale-Bopp, can be many millions of kilometers.
The nuclei of comets range in size from a few miles (kilometers) or smaller to over ~160 kilometers in diameter. The brightness of the comet is not always directly related
to the size of the nucleus. This is because typically only a fraction of the surface of a comet's nucleus is active. It is possible that this comet has a small nucleus with most of its surface emitting dust and gas. It is also possible that this comet has a large nucleus with only a small active region. Recent estimates of the nucleus size, based on the amount of material being generated by the comet, put the nucleus at about 40 km (about 25 miles) in diameter. However, this is just an educated guess. For comparison, Comet Halley's nucleus, which we did see via spacecraft observations, looked like an irregular potato 8 x 8 x 16 km.
Ratio data taken in March 1995 with the IRAM interferometer also confirm this size since the signal they detected can by accounted for by a 380K sphere with a diameter of 45 km emitting radiation.

Activity of the comet
The comet was active (i.e. producing gas and dust) when
discovered at 7.1 AU, and there was a coma even in the
pre-discovery image from 13.0 AU. Since the amount of solar
radiation is too low at these distances to heat water-ice enough
to initiate sublimation, it was known early on that other volatiles
were playing a role in the activity. The first detection of CO
was made at 6.6AU by Jewitt, with the comet producing
1300 kg/sec of CO gas. In comparison, comet P/Halley attained a water production rate this high only near 2 AU! Between 4.7-4.1 AU the CO production flattened out, and the production of OH (a fragment of the water molecule) increased . As the comet neared 2.4 AU, there was a rapid increase in water production. One of the first molecules usually detected in a comet is the CN molecule, which is typically released along with the sublimating water. In the case of comet P/Halley, this was first seen at the unusually large distance of 4.8 AU from the sun. In contrast, during August of 1996 the production of the CN molecule was measured at a distance of 6.8 AU. There is an indication from observations from Lowell Observatory in Arizona that in March 1996the gas progection began to level off. Additionally, dust production was increasing at a slower rate than the trends in mid-January and February.
The comet had a very big dust tail unlike the comet C/1996B2 - Hyakutake, which displayed a prominent gas tail.

Very interesting observations of the Dust Jets and Nucleus Rotation were received.
Beginning around May 1996 observers began reporting strong jetting activity from the comet - consisting of several straight jets, which changed very little in appearance
throughout summer and fall. Jet Propulsion Laboratory scientist, Z. Sekanina in 1997 has recently interpreted the jets as being boundaries of fan-shaped formations where dust is being ejected from 3-4 discrete active sources on the rotating nucleus. In the image at the right, a technique similar to that used by H. Weaver to estimate the nucleus size was used on this image of comet Hale-Bopp obtained usingthe UH 2.2m telescope on Mauna Kea. By removing the smooth coma variation, the jets stand out dramatically. Observations of the dust jets were made in January and February, 1997 by astronomer J. Lecacheux at the Observatory of Paris showed that the comet nucleus is rotating with a period near 11.5 hours. Observations by Italian astronomers the following month confirmed this rotation period by looking in the infrared at shells (which are formed by periodic ejection of gas and dust as an active source rotates into and out of the sunlight). The Italian astronomers deduced that the dust was leaving the nucleus at a velocity of about 0.35-0.45 kilometers per second. Results, published on March 11, 1997 by Jorda and others in the International Astronomical Union Circulars, from observations by French and German astronomers using the Pic du Midi telescope in France suggest that the comet may exhibit complex rotation - that the rotation period is changing between 11.2 and 11.6 hours on a timescale of 22 days. This is like a top which has not only a simple rotation about its axis, but which also has a wobble, and may be caused by uneven outgassing from the jets.

Chemistry of the comet
For us it was very interesting to understand that compounds
like HCO+ Sulfur monoxide were found for the first time in
this comet and that organic molecules were in it.
Organic molecules are being detected in both the radio
wavelengths and in the infra-red and are so far providing
exciting information on how similar the comet nucleus
material is to the material in the interstellar medium.
Astronomers managed to determine the chemical
composition of the comet. The comet astronomer Sekanina
said "One can infer that C/1995 O1 might be (relatively) CO-rich and, at the same time, dust-rich. If such CO supplies would last until perihelion, then the comet could indeed become very bright. Yet, it does not have to have an excessively large nucleus." CO is carbon monoxide, which can sublimate (turn from solid to gas) at great distances from the Sun. When comets are closer to the Sun, water ice (H 2 O) is normally the primary component that sublimates.

Molecules discovered in comet Hale-Bopp:

8. Comparison of the chemical compound of the comet Hale-Bopp with the comet Hyakutake
If we compare the chemical portrait of both comets, we will see that they had only one similarity. In both of them the contents of water was 100%. The contents of CH 3 OH in the comet Hale-Bopp is 2 times bigger than the contents in the comet Hyakutake. The content of CO is also bigger. But in the other way the comet Hyakutake contained
NH 3 , CO 2 , H 2 CO, CH 4 . These substances couldn't be found in Hale-Bopp.
Latest news about the comet:
A group of astronomers led by Alan Stern from South-West Research Institute, USA announced in early June 2000 the discovery of the noble gas argon in Comet Hale-Bopp. The astronomers made the observation on March 29, 1997, by means of an ultraviolet-sensitive instrument on a sounding rocket.
Astronomers from ESO La Silla Observatory took images of the comet during the period February 27 - March 2, 2001. They show that the Comet Hale-Bopp is still active at a distance of about 2,000 million km from the Sun. It is also a very large object, measuring at least 2 million km across

9. How did scientist receive the information :
The information about this comet was received from visual observations, photographical, CCD, photometric and spectroscopic observations and through the space telescope Hubble. To receive information about this comet many astronomers - professional and amateur - observed the comet with telescopes and CCD. They made more than 2000 visual, 3 000 astrometric, 5 000 photographical, CCD spectral and photometrical observations. These observations helped astronomers to make a very detailed picture of the comet.
The orbital elements of the comet were estimated very sharply using 3000 astrometric observations. Thanks to the photometric observations it can be made a rough model of the comet, taking into consideration the size of its coma, tail and nucleus.
The first observations of the comet were made from the observatory La Palma, 1-m Kapteyn Telescope, Hubble Space Telescope, European Southern Observatory 3.5m New Technology Telescope, Canada France Hawaii, 3.6m Telescope, UH 2.2m Telescope on Mauna Kea. We think that astronomers who examined the comet most are from: Isaac Newton Group of Telescopes, La Palma, European Southern Observatory (ESO), Hubble Space Telescope (HST), Jet Propulsion Laboratory (JPL), South African Astronomical Observatory, University of Hawaii, USA.
Except astronomers, many amateur astronomers took part in the observations of the comet and they made thousands of photographes. The amateur astronomers made thousands of valuations of the comet and this allowed very sharp determination of the absolute magnitude and the photometrical parameters of the comet.
After the combination of professional astronomers' observations, examinations of the space telescope Hubble and amateur comet observations scientists made a very precise portrait of this comet.

10. We make a picture of the comet:
When the comet could be observed on the night sky we were 11 years old and we haven't made observations of the comet, so we will show visual, photographical and CCD observations made by our leader Veselka Radeva.

Visual Observations of the comet:
The members of our team observed the comet but the visual observations are made by the leader of the group Veselka Radeva. Here is her description of the comet:"I have been observing comets since 1982. The comet Hale-Bopp is the brightest and the most beautiful comet I have ever seen. It made an incredible space show- we observed it when it was faint and with a small tail, then it became a wonderful ornament of the night sky, with a large and bright coma and with a blue plasma and a white-yellow dust tail. This comet was perfect for teaching young comet observers who saw the "comet astronomy" alive and they used the comet to learn the "comet anatomy".
The first visual observations of the bright comet, made by our leaderVeselka Radeva show that on 1996 04 25.08 the comet had aperant magnitude 8m.4, when it was at a distance of about 5 AU from the Earth and the Sun. With its approach to the Sun the coma increased its brightness and astronomers explained this with the high activity of the comet's nucleus. The last observations of the comet made by our leader are made on 1997 03 17.70, when the comet had visual magnitude -0 m.9 and was at a distance of 1.330 AU from the Earth and of 0.950 AU from the Sun.

Photographical observations of the comet:
The photographes of the comet are made by our leader Veselka Radeva with 50/70 cm Schmidt telescope in the National Astronomical Observatory-Rozhen, Bulgaria with astroplate ZU21

The best Bulgarian photographes of the comet:
1. The best astrophotogroup Sky Treasure Hunters from Silistra, Bulgaria

2. Tashko Valchev- from National Astronomical Observatory-Rozhen, Bulgaria

Fig 29: Comet Hale - Bopp from NAO-Rozhen

CCD observstions of the comet
We show you the first CDD observation of the comet. It was made by Tashko Valchev with 2-m telescope from NAO-Rozhen

Only for teachers and students who love astronomy and find astronomical observations exciting and impressive!
The exercise can be used during the lessons for observations of sky objects or to be included in the observational programmes of student astronomical clubs.
It is suitable for students who are really interested in astronomy and
like to make astronomical observations.

11. Exercise: Determination of the size of the coma and the tail using visual observations of the comet Hale-Bopp
The main goal of this exercise is students to learn how they can give valuaton of the brightness of the comet and determine its apperant magnitude (m k ), the size of the coma (D) and the length of the tail (Tail) in arc units. The given exercise is the base of amateur comet observations.
Introduction:
In the observations of comets the observations made by amateur astronomers are very important. These observations, together with the observations of professional astronomers, make a full and correct picture of the observed comet.
Necessary information: To be made a correct observation, students should use ephemeredes of the comet, sky maps and catalogs and if it is necessary binoculars and telescopes.

Exercise: Determination of the visual magnitude of the comet, the size of the coma and the tail
Preliminary preparation: using the ephemeredes of the comet, draw the comet on a sky map. Find a star that is brighter and a star that is fainter than the comet.
Task1: Using the sky map, find where on the night sky the comet is. If you use a binocular, try the stars for comparison to be in your field. Unfocus your binocular till the stars look as big as the unfocused image of the comet. Divide the distance between the faint and the bright star into 5. Compare the brightness of the comet first with the brighter star and than with the fainter one.
Example : Let's see how our leader Veselka Radeva made her observation.
1. The time of the observations should be written - date, hour and minutes.
The observations were made on 20.11.1996 at 18h00min UT. The comet was in the constellation Ophiuci.
2 .There are 2 stars used for comparison: the brighter star will be Gamma Oph ( m v=3. 75), ad the fainter one will be 61( m v=6.13). We divide the interval of brightness between both stars into 5 parts.
3 .Compare independently the unfocused image first with the brighter and then with the fainter star. We write the result: a3k2b
This means that the bright star (a) is 3 times brighter that the comet (k) and the comet (k) is 2 times brighter than the faint star (b). When we know that the magnitude of Vega is 0 m .03 and the magnitude of Deneb is 1 m .33 we calculate the difference of magnitudes of the stars used for comparison:
= 6.13 - 3.75 =2.38
We divide into 5 and calculate that one part= 0 m.476
According to our estimation the comet is 2 times brighter that the faint star. Thus
m b - 2 x 0 m.5 = 6,13 - 2 õ 0.5 = 5 m.13 or ma + 3 x 0 m.5 = 3,75 + 3 õ 0.5 = 5 m.25
We find the following value: (5 m.25 + 5 m.13)/2= 5 m.19
Therefore the value of the visual magnitude of the comet is 5 m.19
4.The estimation is made with a binocular with aperture 50mm and magnification 10mm and we will have to make corrections:
For binoculars the correction is a.(D-Do), where D is the aperture of the binocular or telescope in cm (here it is 5cm), Do = 6.78 , a= 0.066 for refractors and a= 0.019 for reflectors.
In this case the correction will be 0.066.(5 - 6.78) = - 0.12
We calculate the difference between mk and the correction -0.12 and that is the corrected for the aperture value for the visual magnitude of the comet.
5 m.19- (-0.12) = 5 m.32
mk = 5 ^m.3

Task2: There are 2 methods for determination of the size of the coma and the tail of the comet.
1. The Method of "Dreifa' is based on the fact that when the telescope doesn't move, the comet will slowly cross the area of the eyepiece because of the movement of the sky sphere. Using an eyepiece with a cross, the comet has to move over one of the threads. Using a stopwatch it is determined the time t , for which the comet or the tail crosses the perpendicular thread.
The diameter of the coma can be calculated on the formula:
D = 0.25 cos t cos ( )
where is the comet's declination.
This method cannot be used for comets that have large size (the diameter of the head or the tail is up to 5').
2. The second method uses the fact that the diameter of the coma is determined using the degree distance between the stars that can be seen in the eyepiece. Using maps and atlases with large scales the arc distances between the stars are determined and they are compared with the visual size of the comet. This method is used for comets that are larger than 5'.
These 2 methods are used for determination of the size of the comet's tail.
Example: Using visual observations made by Veselka Radeva made on 20.11.1996, the size of the coma is measured on the 2nd method and it is 9 arc minutes. The length of the tail is 50 arc seconds.

12. Conclusion
The comet Hale-Bopp, about which we learned a lot during our astroresearch work, is one of the most beautiful and observed comets. It will be remembered as one of the most impressive comets that humanity has observed because of its long tail and high brightness. Each photo that we found is so beautiful that we wanted to work over the project more and more. During our research work we understood the importance of many data from many observatories so a scientifical portrait of this object can be made. It is true that comets are the most beautiful objects in the Solar system and our comet Hale-Bopp is one of the biggest and the most beautiful members of the comet's family.

References:
Meeting with comets, Nikola Nikolov, Valeri Golev, V.Racheva
The Comet's tail, Newsletter of the Comet Section of the British Astronomical Association, Volume 9,¹ 1,2002 April
The Comet's tail, Newsletter of the Comet Section of the British Astronomical Association, Volume 9,¹ 2,2002 October
International Comet Quarterly, Volume number 95, July 1995
http://cfa-www.harvard.edu/iau/info/Astrometry.html
http://cfa-www.harvard.edu/iau/mpc.html
http://cfa-www.harvard.edu/icq/icq.html
http://encke.jpl.nasa.gov/
http://cfa-www.harvard.edu/icq/CometMags.html
http://cfa-www.harvard.edu/icq/cometobs.html
http://cfa-www.harvard.edu/iau/Ephemerides/Comets/index.html
http://cfa-www.harvard.edu/iau/Ephemerides/Comets/1995O1.html

For Contacts:
Student:
Parvoleta Marinova
Leader:
Veselka Radeva - Astronomical Observatory-Varna, Bulgaria
radevi@mail.varna.techno-link.com
The student is from the Astronomical courses made by the Astronomical Observatory and Planetarium "N.Kopernicus"-Varna

Fig.1 Photo by Bob Jen
(www.comet-track.com)

Fig.4:Photo by Tashko Valchev,
2-m telescope +ST6 in
NAO-Rozhen, Bulgaria

Fig.5:Photo Tashko Valchev,
2-m telescope +ST6 in
NAO-Rozhen, Bulgaria

Fig. 12:
Comet Hyakutake

Fig. 13: Comet Hale-Bopp

Fig.14: Space
telescope Hubble

Fig.15: European Southern Observatory

Student:

Parvoleta Marinova