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The fast pulse of the RR LYRAES: an amateur program tracks the quirks of the oldest standard-candle stars in the universe.

If you ever took an astronomy course, you probably learned about the role of the pulsing RR Lyrae stars in astronomers' long and heroic construction of the cosmic distance scale. A few amateurs know these stars in another context: as a pleasant way to pass a summer's night.

All RR Lyrae variables of the type considered here have nearly the same absolute visual magnitude, +0.75. That means they're 40 to 50 times as luminous as the Sun--not nearly as bright as the giant Cepheid variables, those more famous standard distance markers that can be seen much farther away.

RR Lyraes are extremely old, however, so they mark ancient populations of stars that lack any Cepheids--such as globular clusters and the Milky Way's outer halo. A star becomes an RR Lyrae very late in its life if it began as a yellow dwarf with only about 80% the mass of the Sun. After such a star finally ages off the main sequence, completes its red-giant phase, and evolves onto the horizontal branch in the Hertzsprung-Russell diagram (shown at left), it spends a while pulsing rapidly before advancing to its final death throes.

The pulses amount to about one magnitude, easy enough to measure by eye at the eyepiece. The stars have periods of about half a day, and their rise from minimum to maximum light can take an hour or less as you watch.

For the last half century, a small, select group of amateur variable-star observers have kept nearby RR Lyraes--visitors from the galaxy's halo that are just passing through--under regular scrutiny. The idea is to time when their maxima occur, in particular to follow strange irregularities that many RR Lyraes display known as Blazhko cycles. These oddly periodic changes in the strength and timing of maximum light were discovered 110 years ago by the Russian astronomer Sergei Blazhko and remain pretty much unexplained.

All such star-timing work is done now by CCD photometry, says Gerry Samolyk, who runs the Short Period Pulsator Section of the American Association of Variable Star Observers (AAVSO). An automated camera on a telescope, clicking away tirelessly while you observe something else (or snooze indoors), results in far more accurate brightness measurements, and therefore timings, than eyeball estimates used to do. And an automated camera can record a high-quality light curve of a star's entire pulse, which may be rather different than the previous pulse just hours earlier.

To get a taste for these stars, on these pages are comparison-star charts for three RR Lyraes in easy reach of amateur telescopes.

Set up at least an hour before a predicted maximum. About every 10 minutes, make a very careful estimate of the variable's exact brightness with respect to comparison stars around it, labeled on the charts with their visual magnitudes to the nearest tenth with the decimal points omitted. Do not let yourself be influenced by what you saw a few minutes ago; each estimate must be unbiased and independent. You'll likely see faster action than amateurs usually observe anywhere else beyond the solar system.

Watch out; this project could get you hooked enough to buy an astrocamera, learn how to do automated photometry, and change your nights forevermore. If you'd like to dive in, read the AAVSO's CCD Photometry Guide at aavso.org/ ccd-photometry-guide and then write to Samolyk care of aavso@aavso.org.

Listed below are Universal dates (bold) and times (in hours) for maxima predicted to happen well within dark hours for at least part of the continental United States and southern Canada. Times are rounded to the nearest 0.5 hour to avoid biasing what you see.

Maxima of VX Herculis

Period = 0.4553595 day

May 1, 4.5; 4, 9.0; 5, 7.0; 6, 5.0; 9, 9.5; 10, 7.0; 11, 5.0; 12, 3.0; 14, 9.5; 15, 7.5; 16, 5.0; 17, 3.0; 19, 9.5; 20, 7.5; 21, 5.5; 22, 3.5; 25, 8.0; 26, 5.5; 27, 3.5; 30, 8.0; 31, 6.0.

June 1, 3.5; 4, 8.0; 5, 6.0; 6, 4.0; 9, 8.5; 10, 6.5; 11, 4.0; 14, 8.5; 15, 6.5; 16, 4.5; 20, 6.5; 21, 4.5; 25, 7.0; 26, 5.0; 30, 7.0.

July 1, 5.0; 2, 3.0; 5, 7.5; 6, 5.0; 10, 7.5; 11, 5.5; 12, 3.5; 15, 8.0; 16, 5.5; 17, 3.5; 20, 8.0; 21, 6.0; 22, 3.5; 25, 8.0; 26, 6.0; 27,4.0; 30,8.5; 31,6.5.

Maxima of XZ Draconis

Period = 0.467497 day

May 1, 4.5; 2, 3.5; 7, 9.0; 8, 8.0; 9, 7.0; 10, 6.0,11, 4.5; 12, 3.5; 17, 9.5; 18, 8.0; 19, 7.0; 20, 6.0; 21, 5.0; 22, 3.5; 27, 9.5; 28, 8.5; 29, 7.0; 30, 6.0; 31, 5.0.

June 1, 4.0; 7, 8.5; 8, 7.5; 9, 6.0; 10, 5.0; 11, 4.0; 12, 3.0; 17, 8.5; 18, 7.5; 19, 6.5; 20, 5.5; 21, 4.0; 27, 9.0; 28, 7.5; 29, 6.5; 30, 5.5.

July 1, 4.5; 7, 9.0; 8, 8.0; 9, 6.5; 10, 5.5; 11, 4.5; 12, 3.5; 17, 9.0; 18, 8.0; 19, 7.0; 20, 5.5; 21, 4.5; 22, 3.5; 27, 9.0; 28, 8.0; 29, 7.0; 30, 6.0; 31, 4.5.

Maxima of XZ Cygni

Period = 0.466473 day

May 3, 9.5; 4, 8.0; 5, 6.5; 6, 4.5; 10, 9.5; 11, 8.0; 12, 6.0; 13, 4.5; 17, 9.5; 18, 8.0; 19, 6.0; 20, 4.5; 24, 9.5; 25, 8.0; 26, 6.0; 27, 4.5; 31, 9.5.

June 1, 8.0; 2, 6.0; 3, 4.5; 4, 3.0; 8, 7.5; 9, 6.0; 10, 4.5; 11, 3.0; 15, 7.5; 16, 6.0; 17, 4.5; 22, 7.5; 23, 6.0; 24, 4.5; 28, 9.5; 29, 7.5; 30, 6.0.

July 1, 4.5; 6, 7.5; 7, 6.0; 8, 4.5; 13, 7.5; 14, 6.0; 15, 4.5; 20, 7.5; 21, 6.0; 22, 4.5; 26, 9.0; 27, 7.5; 28, 6.0; 29, 4.5.

Lunar Occultation

Late on the night of May 31 June 1, telescope users in most of the U.S. and Canada can watch the dark limb of the first-quarter Moon occult Rho Leonis, magnitude 3.8. Times for hundreds of cities and towns are at lunar-occultations. com/iota/bstar/0601 zc1547.htm. The star is binary, magnitudes 4.3 and 5.4, but with its separation of only 0.1", the two steps will happen only about 0.2 second apart or less at most locations.

Jupiter's Red Spot

The Great Red Spot continues to be unusually colorful and visible. Here are the dates and times (UT) when it should cross Jupiter's central meridian:

June 1, 7:25, 17:21; 2, 3:16, 13:12, 23:08; 3,9:04,18:59; 4,4:55,14:51; 5, 0:46, 10:42, 20:38; 6, 6:34, 16:29; 7, 2:25, 12:21, 22:16; 8, 8:12, 18:08; 9, 4:04, 13:59, 23:55; 10, 9:51, 19:46; 11, 5:42, 15:38; 12, 1:34, 11:29, 21:25; 13, 7:21, 17:17; 14, 3:12, 13:08, 23:04; 15, 9:00, 18:55; 16, 4:51, 14:47; 17, 0:43, 10:38, 20:34; 18, 6:30, 16:25; 19, 2:21, 12:17, 22:13; 20, 8:08, 18:04; 21, 4:00, 13:56, 23:51; 22, 9:47, 19:43; 23, 5:39, 15:35; 24, 1:30, 11:26, 21:22; 25, 7:18, 17:13; 26, 3:09, 13:05, 23:01; 27, 8:56, 18:52; 28, 4:48, 14:44 29, 0:39, 10:35, 20:31; 30, 6:27, 16:23.

Caption: Above: The globular cluster M5, located 25,000 light-years away in Serpens, is nearly 13 billion years old--plenty old enough for stars a little less massive than the Sun to have become fast-pulsing RR Lyrae variables. These are the green dots in the color-magnitude diagram of 15,000 of the cluster's stars. Some free-floating RR Lyraes are much closer to us and in easy reach of amateur scopes.

Caption: Above: Light curves of five of XZ Cygni's pulses, recorded by amateurs using CCDimage photometry. XZ Cygni's pulsation period is very stable, but individual pulses wander around in amplitude and shape. This graph covers the star's entire pulsation period of 11 hours 11 minutes 43 seconds.

Caption: The wavy lines represent Jupiter's four big satellites. The central vertical band is Jupiter itself. Each gray or black horizontal band is one day, from Oh (upper edge of band) to 24h UT (GMT). UT dates are at left. Slide a paper's edge down to your date and time, and read across to see the satellites' positions east or west of Jupiter.
Phenomena of Jupiter's Moons, June 2017

June 1    9:22    I.Oc.D
          12:39   I.Ec.R
June 2    5:18    II.Tr.I
          6:38    I.Tr.I
          7:31    II.Sh.I
          7:42    I.Sh.I
          7:46    II.Tr.E
          8:49    I.Tr.E
          9:52    I.Sh.E
          9:56    II.Sh.E
June 3    3:50    I.Oc.D
          7:08    I.Ec.R
          21:51   III.Tr.I
          23:59   II.Oc.D
June 4    0:18    III.Tr.E
          1:06    I.Tr.I
          2:11    I.Sh.I
          2:21    III.Sh.I
          3:16    I.Tr.E
          4:21    I.Sh.E
          4:35    II.Ec.R
          4:37    III.Sh.E
          22:17   I.Oc.O
June 5    1:37    I.Ec.R
          18:31   II.Tr.I
          19:33   I.Tr.I
          20:39   I.Sh.I
          20:49   II.Sh.I
          21:00   II.Tr.E
          21:44   I.Tr.E
          22:49   I.Sh.E
          23:14   II.Sh.E
June 6    16:45   I.Oc.D
          20:05   I.Ec.R
June 7    11:49   III.Oc.D
          13:12   II.Oc.D
          14:01   I.Tr.I
          14:19   III.Oc.R
          15:08   I.Sh.I
          16:12   I.Tr.E
          16:32   III.Ec.D
          17:18   I.Sh.E
          17:52   II.Ec.R
          18:49   III.Ec.R
June 8    11:13   I.Oc.D
          14:34   I.Ec.R
June 9    7:46    II.Tr.I
          8:29    I.Tr.I
          9:36    I.Sh.I
          10:09   II.Sh.I
          10:15   II.Tr.E
          10:39   I.Tr.E
          11:47   I.Sh.E
          12:33   II.Sh.E
June 10   5:40    I.Oc.D
          9:03    I.Ec.R
June 11   1:32    III.Tr.I
          2:25    II.Oc.D
          2:56    I.Tr.I
          4:00    III.Tr.E
          4:05    I.Sh.I
          5:07    I.Tr.E
          6:15    I.Sh.E
          6:20    III.Sh.I
          7:10    II.Ec.R
          8:35    III.Sh.E
June 12   0:08    I.Oc.D
          3:32    I.Ec.R
          21:01   II.Tr.I
          21:24   I.Tr.I
          22:34   I.Sh.I
          23:27   II.Sh.I
          23:29   II.Tr.E
          23:35   I.Tr.E
June 13   0:44    I.Sh.E
          1:51    II.Sh.E
          18:36   I.Oc.D
          22:01   I.Ec.R
June 14   15:33   III.Oc.D
          15:39   II.Oc.D
          15:52   I.Tr.I
          17:02   I.Sh.I
          18:03   I.Tr.E
          18:05   III.Oc.R
          19:12   I.Sh.E
          20:27   II.Ec.R
          20:32   III.Ec.D
          22:48   III.Ec.R
June 15   13:04   I.Oc.D
          16:29   I.Ec.R
June 16   10:17   II.Tr.I
          10:20   I.Tr.I
          11:31   I.Sh.I
          12:31   I.Tr.E
          12:46   II.Tr.E
          12:46   II.Sh.I
          13:41   I.Sh.E
          15:10   II.Sh.E
June 17   7:32    I.Oc.D
          10:58   I.Ec.R
June 18   4:48    I.Tr.I
          4:53    II.Oc.D
          5:17    III.Tr.I
          6:00    I.Sh.I
          6:59    I.Tr.E
          7:48    III.Tr.E
          8:10    I.Sh.E
          9:44    II.Ec.R
          10:19   III.Sh.I
          12:33   III.Sh.E
June 19   2:01    I.Oc.D
          5:27    I.Ec.R
          23:16   I.Tr.I
          23:32   II.Tr.I
June 20   0:28    I.Sh.I
          1:27    I.Tr.E
          2:01    II.Tr.E
          2:04    II.Sh.I
          2:38    I.Sh.E
          4:28    II.Sh.E
          20:29   I.Oc.D
          23:56   I.Ec.R
June 21   17:44   I.Tr.I
          18:08   II.Oc.D
          18:57   I.Sh.I
          19:21   III.Oc.D
          19:55   I.Tr.E
          20:37   II.Ec.D
          20:37   II.Oc.R
          21:07   I.Sh.E
          21:54   III.Oc.R
          23:01   II.Ec.R
June 22   0:31    III.Ec.D
          2:47    III.Ec.R
          14:57   I.Oc.D
          18:25   I.Ec.R
June 23   12:12   I.Tr.I
          12:49   II.Tr.I
          13:26   I.Sh.I
          14:23   I.Tr.E
          15:19   II.Tr.E
          15:24   II.Sh.I
          15:36   I.Sh.E
          17:47   II.Sh.E
June 24   9:26    I.Oc.D
          12:53   I.Ec.R
June 25   6:40    I.Tr.I
          7:24    II.Oc.D
          7:54    I.Sh.I
          8:51    I.Tr.E
          9:08    III.Tr.I
          9:53    II.Oc.R
          9:54    II.Ec.D
          10:04   I.Sh.E
          11:40   III.Tr.E
          12:18   II.Ec.R
          14:19   III.Sh.I
          16:32   III.Sh.E
June 26   3:54    I.Oc.D
          7:22    I.Ec.R
June 27   1:09    I.Tr.I
          2:06    II.Tr.I
          2:23    I.Sh.I
          3:20    I.Tr.E
          4:33    I.Sh.E
          4:36    II.Tr.E
          4:42    II.Sh.I
          7:05    II.Sh.E
          22:22   I.Oc.D
June 28   1:51    I.Ec.R
          19:37   I.Tr.I
          20:40   II.Oc.D
          20:52   I.Sh.I
          21:48   I.Tr.E
          23:01   I.Sh.E
          23:09   II.Oc.R
          23:12   II.Ec.D
          23:13   III.Oc.D
June 29   1:35    II.Ec.R
          1:48    III.Oc.R
          4:30    III.Ec.D
          6:45    III.Ec.R
          16:51   I.Oc.D
          20:20   I.Ec.R
June 30   14:06   I.Tr.I
          15:20   I.Sh.I
          15:24   II.Tr.I
          16:16   I.Tr.E
          17:30   I.Sh.E
          17:54   II.Tr.E
          18:01   II.Sh.I
          20:24   II.Sh.E

Every day, interesting events happen between Jupiter's satellites and
the planet's disk or shadow. The first columns give the date and mid/
time of the event, in Universal Time (which is 5 hours ahead of
Eastern Standard Time). Next is the satellite involved: I for lo. II
Europa, III Ganymede, or IV Callisto. Next is the type of event: Oc
for an occultation of the satellite behind Jupiter's limb, Ec for an
eclipse by Jupiter's shadow, Tr for a transit across the planet's
face, or Sh for the satellite casting its own shadow onto Jupiter. An
occultation or eclipse begins when the satellite disappears (D) and
ends when it reappears (Ft). A transit or shadow passage begins at
ingress (I) and ends at egress (E), Each event is gradual, taking up
to several minutes. Predictions courtesy IMCCE /Paris Observatory.
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Title Annotation:Celestial Calendar
Author:MacRobert, Alan
Publication:Sky & Telescope
Date:Jun 1, 2017
Words:2738
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