ORIONIDS: ORIGINS, CHARACTERISTICS, WHEN AND HOW TO OBSERVE THEM - ARTE - 2025

The Orionids are known as a meteor shower, visible in the night sky from early October to early November, radiating from the constellation Orion for which they are named.

The sight of a shooting star and the brief trail it traces in the night sky has fascinated all observers since ancient times, but the origin of these fast and fleeting visitors was not clear until the 19th century.

Figure 1. The meteor shower known as the Orionids. Source: Wikimedia Commons.Brocken Inaglory.

Despite being called "stars", they have nothing to do with celestial bodies like the Sun. Shooting stars or meteors have their origin in remnants of materials found throughout the Solar System.

These are the remains of comets and asteroids that have been fragmented due to gravitational interaction, which is also responsible for keeping them in orbit.

As the Earth moves, it encounters this debris. When it comes across a high density of debris, these manage to enter the atmosphere at high speed, ionize the gases present and produce the characteristic light trail. Then - in most cases - they disintegrate due to friction.

The Orionids are nothing less than the remains that Halley, the most famous of all comets, has left on his visits to these parts.

Aside from being the father of the Orionids, Halley's Comet is also responsible for another very striking meteor shower: the eta-aquarids in the constellation Aquarius, visible between April and May of each year.

Due to their location, the Orionids can be admired by inhabitants of both hemispheres, as long as the sky is clear and the moon is low on the horizon. In addition, the presence of Orion, the celestial hunter and the other constellations and planets visible at that time, in themselves guarantee a truly impressive view of the night sky.

origins

The size of the debris left by comets and asteroids is highly variable, from fine dust particles 1 microns - a millionth of a meter - in diameter, to really large fragments kilometers wide.

The Halley is a periodic comet whose last visit was recorded in 1986 and which is expected to return in 2061. It was identified and studied by the English astronomer Edmund Halley in 1705, but it was known long before, being the best documented of all kites.

Figure 2. Halley's Comet against the background of the Milky Way. Source: Wikimedia Commons. Kuiper Airborne Observatory, C141 aircraft April 8/9, 1986, New Zealand Expedition.

As it approaches the Sun, the radiation heats the comet to vaporize a part. In the process, ionized atoms and molecules are released, which emit a spectrum. Through analysis, the scientists recognized elements such as hydrogen, carbon and nitrogen and their compounds: ammonia, water and carbon dioxide, forming part of the comet.

This relationship between meteor showers, comets and asteroids was not evident for some time. The existence of shooting stars was attributed to atmospheric phenomena and not to interactions of the Earth with other celestial objects.

But a surprising and unexpected phenomenon aroused people's curiosity to know the true origin of the meteors: the great Leonid shower of November 1833, with hundreds of thousands of meteors visible in a single night.

Decades after this event, the Italian astronomer Giovanni Schiaparelli found the definitive link between the orbits of comets and the meteor showers, when he verified that the orbit of Comet Tempel-Tuttle coincided with the Leonids. Whenever the comet arrives in the vicinity every 33 years, the Leonids tend to experience intense activity.

Main features

The Orionids, as well as the other major meteor showers, such as the Perseids, the Lyridids, the Geminids and the Quadrantids, among others, are typical of certain times of the year.

In this case, the Orionids begin with the month of October and continue until the beginning of November, the maximum of activity usually occurs in the third week of October, around the 21 of that month. Meteors have a striking yellowish-green color.

Radiant

The Orionids seem to come from a point in the constellation Orion, the hunter. This point is known as the radiant of the meteor shower, which is simply a perspective effect, since the trajectories of the meteors, being parallel, appear to converge at that point.

Zenith rate

Another important factor astronomers use to describe rainfall is the hourly zenith rate, zenith rhythm or THZ, which is the number of meteors per hour under ideal visibility conditions - dark, clear skies and no visible moon.

On average, it is estimated that the Orionids have a zenith rate of 20 - 25 meteors per hour, although when the Earth encounters a large amount of debris left by Halley in previous visits, the THZ reaches up to 50 meteors / hour, with speeds in the range of 60-66 km / second each.

Population index

Finally, the population index describes the brightness of the contrails left by the swarm. It is not easy to quantify, because it depends among several factors, on the mass and speed of the meteors.

When and how to observe them

Orionids are very well observed during the early morning, between 2 and 3 hours before sunrise. Meteor showers do not stop during the day, as revealed by observations made with radar, but unless it is a large fireball, they are hardly seen in daylight hours.

It is a fact that during the course of the same night, the meteors become more and more numerous as time passes. On average, before sunrise you can see twice as many meteors as at sunset, so it is recommended to observe them at these times.

Meteors can actually come from anywhere, but it is after midnight that the Earth meets them head-on, rather than waiting for them to reach ours from behind.

In addition, the meteors observed before midnight seem slower, because the relative speed between two mobiles in the same direction is the subtraction of both speeds, while in opposite directions they add up. An example immediately.

Relative velocity of meteors

Suppose that a fragment moving at 40 km / s meets the Earth before midnight. In this case both Earth and fragment follow the same direction.

Knowing that the Earth has an approximate speed of 30 km per second, the relative speed will be 40-30 km / s = 10 km / s. Therefore this meteor is seen coming at 10 km / s.

On the other hand, before sunrise, when the Earth meets the meteors head-on, this speed is 40 + 30 = 70 km / s and we would see the shooting star coming with a speed 7 times higher.

Figure 3. Relative velocity of meteors. Source: Nasa Science, at science.nasa.gov.

Recommendations to see them

Meteor showers are best seen with the naked eye, therefore binoculars and telescopes are not necessary when viewing them is the only objective. Basically what is required is the patience to scan the sky and wait for the meteors to appear. You have to give yourself time in sight to adjust to the darkness.

However, the sky at the time of year that the Orionids appear is rich in interesting objects that are worth observing with instruments: stars of the first magnitude, nebulae and planets. The most interesting ones are mentioned below.

The radiant of the Orionids is near Betelgeuse, the red giant of Orion, a spectacle in itself, although it is not necessary to look exclusively there to appreciate the rain, since the best thing is to look around all the surroundings.

On the other hand, it is convenient to wait for the radiant to be more or less high above the horizon, and the most comfortable way is to sit on an extendable chair or lie directly on the floor.

Also, as observing the sky takes time, it is a good idea to have:

-Blankets.

-Comfortable clothes.

-Foods.

-A thermos with coffee, tea or hot chocolate.

-Lantern.

-Insect repellent.

-Smartphone with sky maps.

Finally, to photograph the event, the most appropriate equipment is a reflex camera equipped with a tripod and self-timer.

Astronomical objects visible in October

Minor rains in October

Apart from the Orionids, there is another minor shower of the same date: the Epsilon-Geminids, with radiant near the star Pollux in the constellation Gemini and the Draconids or Giacobinids (for the associated comet) in the Dragon.

To locate constellations, star names, and other important astronomical objects, there are apps that offer sky maps. The internet abounds with updated information with the best moments to observe and innumerable details about the sky and objects visible at night.

Interesting astronomical objects visible to the naked eye

The October sky is rich in stars of the first magnitude, visible to the naked eye in a clear sky and far from light pollution. Here is a brief list of the most striking and the constellation to which they belong in parentheses:

-Altair, (Eagle)

-Capella, (Auriga)

-Deneb (Swan)

-Fomalhaut (Piscis australis)

-Betelgeuse (Orion)

-Rigel (Orion)

-Sirio (Can Mayor)

-Canopus (Carina)

-Aldebaran (Taurus)

-The Pleiades or the 7 goats, is a cluster of young stars (Taurus).

Figure 4. The constellation Orion, the hunter. In the center are the three stars that make up the belt. Source: Pixabay.

In addition to the stars, two globular clusters, known as the Perseus double cluster, are called NGC 869 and NGC 884 and are visible in the constellation of the same name, when the sky is very dark and clear.

As for the nebulae, the Orion belt is adorned with the Orion Nebula, visible to the naked eye, while at the equator and in the southern skies you can see the Tarantula Nebula, near the Magellanic Clouds, two small irregular galaxies very close to the Milky Way.

Among the visible planets, Venus, Saturn and Jupiter stand out for their brightness, from the latter the moons can be observed with binoculars or a small telescope, as long as they are not behind the planet.

In the constellation Cassiopea, easily identifiable by its W shape, is the spiral galaxy Andromeda. If the sky is very clear, it can be distinguished with binoculars or better yet, with a telescope.

References

1. American Meteor Society. Major Meteor Showers. Recovered from: amsmeteors.org
2. Maran, S. 2013. Astronomy for Dummies. L Books. chap. Four.
3. POT. Orionids. Recovered from: solarsystem.nasa.gov
4. Oster, L. 1984. Modern Astronomy. Editorial Reverté. 107-111..
5. Pasachoff, J. 1992. Stars and Planets. Peterson Field Guides. 413-418.
6. Sky & Telescope. The Best Meteor Shower in 2019. Recovered from: skyandtelescope.com.
7. Wikipedia. Orionids. Recovered from es.wikipedia.org.