What is a Star?
A star is an astronomical object, like a massive ball, that consists of hydrogen and helium, with enough mass to enable it to sustain nuclear fusion at its core. The best-known star is the Sun, which is the only star in our solar system. The Star we are concerned with for this post is Tarf (β Cancri).
The luminosity of the Sun is measured as 1 on the standard luminosity scale and anything larger than 1 signifies that Star emits more energy than the Sun. Basically the more luminous a star is, the more energy it produces.
Tarf (β Cancri), is cooler and emits less energy than our Sun, but it is still easily recognizable as the brightest star in the constellation of Cancer.
Characteristics of a Star
There are 5 main characteristics that define a star:
- Brightness – luminosity and magnitude
- Mass – using the Sun as the benchmark
- Size – using the Sun as a benchmark
- Color – depends on the temperature of the star
- Surface temperature – measured on the Kelvin scale
The brightness of a star is characterized by two factors: Luminosity and Magnitude:
- Luminosity – is the amount of light that a star radiates as determined by its surface temperature and its size
- Magnitude – the magnitude star factor measures the brightness of that star in two ways, by its ‘apparent magnitude’ and its ‘absolute magnitude’.
The brightness measure, when both the distance and the size of that star are taken into consideration, is represented by the term ‘Apparent Magnitude’.
Whereas, when the true brightness of a star is measured, irrespective of its distance from Earth, it is represented by the term ‘Absolute Magnitude’.
The mass of a star is measured against the mass of the Sun, where the Sun is measured as 1, and said to equal 1 solar mass.
FACT: Solar Mass is a standard unit of mass in astronomy, equal to approximately 2×10 (to the power of 30) kg. It is used as a measure for stars, nebulae, black holes and other solar masses of material.
A star that is two and a half times bigger than the mass of the Sun would be measured as having a mass of 2.5 solar masses.
The measure of mass also takes density into consideration.
Our Sun, and the planet of Jupiter are often used as a benchmark for measuring many objects, like Stars, in the Solar System.
The size of a star is normally compared with the size of our Sun, so if a star was twice the size of our Sun it would be measured as 2 solar radii, and if it was the same size as the Sun it would be measured as 1 solar radii.
Stars that are similar in size may have a different mass measure as density has to be factored into the measurement of mass.
The simple spectrum of color measure for stars is determined by surface temperature.
From a young age we associate temperature with colors. The yellow Sun is hot and the deep blue water is cold.
However the opposite of what we are led to believe about this relationship between color and temperature applies with stars. We normally associate blue as an icy cold color and red as a hot color.
However with stars, the hotter stars have a cooler color and the cooler stars have a warmer color.
This means that a blue supergiant star would be hotter than a red supergiant star, with blue-white, white, yellow and red-orange stars, somewhere in between.
The temperature of a star is measured on a scale known as the ‘Kelvin scale’.
On the Kelvin scale, zero degrees is theoretically absolute and equivalent to -273.15 degrees Celsius.
The approximate temperatures of stars by color range from:
- Hot Stars (Blue) – 50,000 K
- Tarf, Cancri b, (Orange to red) – 3,990K
- The Sun – 5,500 K
- Cooler Stars (Red)- 2,500K
So let’s take a look at one of the cooler and mature Fixed Stars in our night sky.
- The Arabic named Star Al Tarf (Beta Cancri, 17 Cancri)
The Tarf Star– Facts in brief:
What is it?
The Fixed Star named Tarf, beta Cancri, (17 Cancri – its Flamsteed designation) is one of the many bright stars within the well-known and medium sized zodiac Constellation of Cancer.
Tarf, Cancri b, is located on the main outline of the Constellation of Cancer, known as ‘The Celestial Crab’, positioned on one of the legs of the Crab. The name al Tarf, means the end, (presumably referring to the position of the great star Tarf on the leg of the crab outline)
It is the brightest star in this Cancer Constellation (one of the 12 Zodiac Constellations) however with an apparent/visual magnitude of +3.55 it is recognized as the 297th brightest Star in the celestial sky. At this level of luminosity it is relatively faint it is still visible by the naked eye, from Earth.
Al Tarf, its traditional name, is a main sequence giant star that can be described in two ways:
- An orange K-type giant star, of the spectral type K4III Ba1
- A binary star system, with a companion that is a Red Dwarf Star with a fourteenth magnitude rating.
It is a suspected variable giant star, partly visible from both the northern Hemisphere and the Southern Hemisphere, in the night sky, at certain times of year.
It can mainly be seen from the Northern Hemisphere from late autumn until spring, but can also be seen in the summer and autumn months in the Southern Hemisphere. The image of the Crab will appear upside down when viewed in the Southern Hemisphere.
Tarf (β Cancri) in Cancer
Tarf, (b Cnc) is recognized as a binary star (with a fourteenth magnitude companion star). They orbit each other and this orbital period occurs once every 76,000 years. It’s orange in color and is the brightest star in Cancer, with a borderline fourth/third magnitude rating.
- Tarf is the brightest star in Cancer, although it does not have the Bayer allocated alpha name. It has an apparent magnitude of 3.50, located 290 light years (89 parsecs) from our Solar System and visible by naked eye in the night sky.
- It is best seen in the month of March.
The Constellation of Cancer is the 31st largest Constellation overall in the Northern Sky, and visible at latitudes of +90 to -60 degrees. It borders several other well known, and easy to identify Constellations – Leo (to the east) and Gemini (to the west).
The age and size of Tarf (β Cancri)
It is estimated by Hipparcos to be around 2.0 billion years old, although this is not certain, nor is it known how much longer its lifespan will be.
FACT: if a Star has less than 8 solar masses (is not at least 8 times the size of the Sun), it cannot end life in a supernova explosion. As we don’t know the exact size of this Star it is difficult to determine its ending. It could become a hot white dwarf and simply burn out.
Based on its spectral type of K4III the color of this suspected variable giant star, with its Arabic traditional name – Al Tarf – is orange.
This is a giant star that is cooler than our Sun and that has an abundance of barium in its color spectrum. It is therefore also referred to as a cool Barium Star.
It has expanded over time and was estimated, according to the latest Hipparcos data in 2007, to have 61 times the radius of the Sun (expanded to a radius of 61 solar radii, 6,100 % of the radius of the Sun), and is around 122 times bigger than the Sun.
The luminosity of Tarf is not bright enough, with an apparent magnitude of +3.50, for it to be listed as one of 58 bright stars commonly used for celestial navigation and marking our position.
FACT: The Navigational stars are some of the brightest and most easily recognized stars in the night sky. They were used by many ancient civilizations during sea voyages and from land points to mark time, locate their position and plan directions for journeys.
FACT: The 58 recognized celestial navigational stars include 19 first magnitude stars and 38 second-magnitude stars. As a third magnitude star, Tarf does not qualify with its luminosity.
In Greek Mythology
Tarf is positioned within Cancer, which is known as the Crab according to the Greek myth of a crab being cast into the celestial sky. The Constellation of Cancer is one of the first 48 Constellations cataloged by the Greek astronomer Ptolemy.
Characteristics of this orange K-type giant star
Tarf is a binary system, with a variable giant star and a Red Dwarf Star, as its fourteenth magnitude companion.
Its Red Dwarf companion has an angular distance of 29 arcseconds it is estimated to be 2,600 AU distant from beta Cancri, with an orbital period of approximately 76,000 years.
- The main star is designated – WDS JO8165+0911A
- The secondary companion star is designated – WDS JO8165+0911B
Tarf is orbited by an exoplanet that has approximately 7.8 times the mass of Jupiter. Its orbital period is 705 days.
FACT: An exoplanet (interchangeable with the word extrasolar planet) is a planet that evolves around any star other than the Sun.
There is some debate around when to use the word exoplanet and when to use the term extrasolar planet.
It appears to be a matter of choice, however is you check which one the search bodies prefer, Wikipedia prefers ‘exoplanet’ and the Encyclopedia Britannica prefers ‘extrasolar planet’.
The brightness of TARF is only used as the guide point for other Constellations, asterisms or stars, when it is viewed as part of the Zodiac Crab Constellation.
It is also referred to as a variable barium star, with a third magnitude rating (apparent magnitude ranges from 3.50-3.58, and absolute magnitude -1.218), and a surface temperature of around 3990 Kelvin, at the latest estimate, giving it a similar luminosity to the Sun, which is estimated to be 5,775K.
Tarf is very energetic star but cooler and around 871 times brighter than the Sun, with a
- Radial velocity of 22.94km / 14.25 mi per second
- Rotational velocity of 6.9 km / 4.2 mi per second
Altarf (b Cnc) is a Fixed Star and it is visible by naked eye.
The latest Hipparcos distance estimate in 2007 for this orange (almost Red) giant star locates it around 290.44 light years from our solar system (around 89.05 parsecs). It has an exoplanet orbiting it.
FACT: 1 light year equals 0.3066 parsecs
However we now know that everything moves around in space, the planets orbit stars and the stars orbit the galactic center.
Tarf’s Galacto-Centric distance is 24,367.66 light years (7,471.00 parsecs).
It is now located about 19,186,618.58 AU from the Earth/Sun, according to the 2007 Hipparcos distance data.
The proper motion movement of stars away from/to the Sun is measured in milliarcseconds.
If the measure is negative it means the star is moving closer to the Sun, and if the number is positive it means the star is moving away from the Sun.
Tarf is moving -46.82 milliarcseconds east (towards from the Sun) and -49.24 milliarcseconds north (towards the Sun).
The Radial Velocity is the speed at which the star is moving either away from or towards the Sun, and with a projected velocity of 22.94000 km/s it is drifting closer to the Sun. (with an error, parallax, of +/- 0.19km/s).
The ecliptic passes through the Constellation of Cancer, which means it is subject to lunar occultation.
Who named the stars?
In 1603, the German Astronomer – Johann Bayer, systematically assigned names to the brightest stars in each constellation and cataloged them in his Star atlas – ‘Uranometria Omnium Asterismorum’.
The Bayer designations are stellar designations where the stars within Constellations such as Constellation Cancer, are initially identified by a name or a letter from the Greek Alphabet from Alpha through Omega (in order of brightness).
1. The names of the Stars begin with a letter of the Greek alphabet starting with– Alpha, then beta, gamma, etc.
2. Followed by the genitive form of the Cancer parent constellation’s Latin name – ‘Cancri’, sometimes abbreviated to ‘Cnc’.
Johann Bayer did not assign the alpha name to the brightest star in Cancer; instead he assigned the beta name to this star.
The main sequence stars of Cancer are listed by their apparent magnitude (luminosity) from Earth, listed from the brightest to faintest star in decreasing order.
Those with an apparent magnitude of less than 5.00 are:
1. Tarf, Altarf, (beta Cancri, b Cnc) is a binary system star and the most luminous star in Cancer, with a luminous apparent magnitude of 3.53, and 290 light years distant
2. Asellus Australis, (Delta Cnc), a double Star, the second brightest star in Cancer, with a visual/apparent magnitude of 3.94. It is better known than Tarf as it marks the famous open cluster known as the beehive Cluster (Messier 44)
3. Iota Cancri – a double star, the third brightest star with a variable magnitude of 4.03
4. Acubens, Serben (alpha Cnc) – the fourth brightest component in Cancer and is a suspected variable star system, with a visual magnitude of 4.26
5. Ausellus Borealis, Gamma Cnc, a multiple Star System, with an apparent magnitude of 4.66
Where is it in the shape of Cancer?
The outline of the Constellation of Cancer is in the shape of a Crab (a well known symbol of this large zodiacal constellation, and Tarf represents a part of the leg on this figure.
The traditional name ‘Al Tarf’,of the star b Cancri, or Cancri b, has been officially recognized by the International Astronomical Union (IAU).
The Constellation of Cancer has a total of 121 Stars. There are 76 Bayer/Flamsteed named Stars.
Properties of the Tarf (β Cancri) Star
Tarf is a massive star, classified as an orange giant star. It is located approximately 303.41 light years distant from Earth. The Ecliptic runs through the Constellation of Cancer, and as the star named Tarf is close to the ecliptic it is prone to be occulted by the Earth’s Moon.
Tarf, within the Constellation of Cancer, is located towards the center of the Milky Way Galaxy, and roughly bordered by:
- Hydra Constellation – to the South
- Canis Major Constellation – to the Southwest
- Leo Constellation – to the East
- Leo Minor Constellation – to the Northeast
- Lynx Constellation – to the North
- Gemini Constellation– to the West
Generally the rate at which stars evolve depends on their mass. The greater the mass the quicker the star evolves from the main sequence.
The mass of the sun, referred to as its stellar mass, and enumerated as the Sun’s mass as a proportion of solar mass.
The mass of the Tarf star components is much bigger than the Sun however the figure is unknown. It has strong stellar winds which means it can lose mass quickly.
The surface gravity of Tarf is 1.5 cgs.
The Tarf Star has a solar radius of 6,100% times that of the Sun, referred to as 61 solar radii.
How bright is this Star
Tarf, the orange spectral type K4lll giant star, has a brightness that ranks it as a fourth/almost third magnitude luminous star when viewed from Earth.
Its luminosity is the amount of energy emission from this star relative to the Sun.
The brightness of a star as seen from earth is measured by its magnitude of which there are three classifications: Apparent Magnitude, Visual Magnitude and Absolute Magnitude.
The measurements for the brightness of Tarf are:
- An apparent magnitude (also referred to as its visual magnitude) of +3.53
- An absolute magnitude of -1.22
FACT: The Apparent Magnitude is how bright we see a Star from Earth, and the Absolute Magnitude is the Apparent Magnitude of that star from a 10 parsecs distance (32.6 light years), assuming there are no molecular clouds, or dust in the line of sight.
These measurements are based on parallax measurements taken during the Hipparcos mission.
The lower the number of magnitude the brighter the Star.
The color of the Tarf Star
The Tarf Star is classified as a giant Star, which according to its spectral type, K4III Ba1, is an orange to red colored star.
Based on the spectral type of this Star it is estimated to have a surface temperature of around 3,990 degrees Kelvin. This makes it one of the cooler type stars in the Universe, and similar to the surface temperature of the Sun.
Where is it located? –
The distances vary depending on how it’s measured and there are certainly margins of error.
According the latest 2007 Hipparcos measurement the parallax data gave an uncertain result. The Tarf Star is estimated to be around 290 light years (89 parsecs) from our Solar System.
FACT: A parallax is a difference in the apparent position of a star or any solar system object viewed from two different lines of sight. The parallax is measured by the angle between the two lines of sight.
The distance between the Earth and the Sun is known as an Astronomical Unit.
The figure of A.U. is calculated as the number of times that Star is from Earth, in relation to the Sun. It is estimated that the Tarf Star is approximately 19,186618.58A.U. from Earth to Sun in distance.
FACT: All Stars and Planets orbit round a central point, the planets orbit the Sun and the Stars orbit the Galactic Centre.
The Star al Tarf is located approximately24,367.66 light years (7,471.00 parsecs) from the Galactic Center, the center of the galaxy we call the Milky Way.
The Galacto-Centric distance is measured as the distance from that Star to the center of the Galaxy (Sagittarius A).
Fact: the Galactic Center, or Galactic Centre, is a supermassive black hole and the rotational center of our Milky Way galaxy. It is not possible to view it at visible, ultraviolet, or even soft or low-energy X-ray wavelengths because of the molecular clouds of interstellar dust along its line of sight.
Where can it be found?
The Tarf Star is of great interest to amateur astronomers as it is visible with the naked eye in a busy part of the night sky. In the constellation of cancer but also alongside several other constellations of the zodiac.
Co-ordinates of a right ascension, or left ascension and their declination are used to locate all of the notable objects in the sky
The Tarf Star lies at a 08 hours 16 minute right ascension,and a declination of +09, 11’ degrees.
The right ascension – is the angular distance of any sky object along the celestial equator from the March (Spring) equinox.
- If it has a positive number it is east of the March equinox.
The declination – is the angle of the sky object from the celestial equator.
- If it has a positive number it indicates it is located in the Northern Hemisphere
Tarf is located close to the celestial equator and is therefore partly visible from both the northern hemisphere and the southern hemisphere at certain times of year.
The age of the Tarf Star
It is not certain what age the Star Tarf is or how long it is likely to continue to survive, but it has already lost some of its energy and mass
One estimate of the age of this Star is that it is possibly around 2.0 billion years old.
How can you identify the Tarf Star?
The simplest method for spotting any particular Star from Earth is to first of all locate an easy to recognize neighboring Constellation or object in the night sky.
Tarf’s parent constellation is probably one of the easiest to spot as it is formed from some stars in a distinctive pattern- the Celestial Crab that appears to have the main Tarf Star positioned on the leg at the end of the outline.
The Constellation of Cancer is located near the celestial equator and that means it is visible in most of the world.
The best time of year to spot the Tarf star is from the Northern Hemisphere in late winter and early spring. Like all stars, Tarf is constantly on the move.
How do we obtain the images and measurements of Stars from Space?
Images of the different Stars and nebulae have been captured using a professional large telescope, such as the NASA Spitzer Space Telescope, with a near infrared camera and multi-object spectrometer, all covered by Wikipedia sources.
The standard measurement of distance for Stars for decades has been using the ESA Earth-orbiting Hipparcos satellite, which replaced the ESA Gaia mission to chart a three dimensional map of the Milky Way Galaxy.
ESA Gaia measured the positions and radical velocities of around one billion stars in our galaxy.
What is the Milky Way?
The Milky Way is a Spiral Galaxy, containing over 200 billion Stars, and actually forms part of the Constellation of Sagittarius.
The Milky Way itself is not a Constellation of Stars. It is the Galaxy that contains our solar system and it gets its name from the fact that it looks like a hazy swirl or river of milk across the sky, when viewed from earth.
It is made up of gas, interstellar dust and stars, with spiral arms wrapped around it, and a massive black hole in the center of the Galaxy. The dust cover means that it is not possible to see the center of the galaxy.
Not all of the Stars in the Universe are contained within the Milky Way. The Milky Way is at its brightest if looking towards the galactic center in the direction of Sagittarius.
The Stars that make up the Milky Way are many light years away and cannot be individually identified by the naked eye.
FACT: The Star System or Stellar System is a small number of stars that orbit around each other and are bound together by gravity.
When it becomes a large group of stars, again bound together in the same way, by gravity, it is known as a Galaxy or Star Cluster.
Whether they contain small groups of stars or larger groups of start the both come under the classification of ‘Star System’.
The celestial pole
The celestial pole defines the poles of the celestial equatorial coordinate system.
An object at the Celestial pole has a declination of 0 degrees.
- The declinations for the north celestial pole is +90 degrees
- The declinations for the south celestial pole is -90 degrees
The celestial poles are not permanently in a fixed position against the background of the stars as everything moves in Space.
The Tarf Star is located in the Southern celestial sky at a +09, 11’ degrees declination, and an average 08 hours, 16 min right ascension.
Fun Facts about Stars – Did you know that?
- There are around 6,000 stars visible by the naked eye from Earth
- Cancer is one of the faintest of the 12 Constellations of the zodiac
- Tarf is sometimes referred to as HD 69267
- It is listed as HIP 40526 in the Hipparcos Star Catalogue
- Tarf’s star type is known as a ‘High proper-motion Star’
- Another star designation for Altarf is SAO 116569
- After the Sun, Sirius is the brightest star in the sky with an apparent magnitude of -1.46
- The Star we call The Sun does not belong to any constellation
- The planet Jupiter is often cited when making size comparisons between planets or stars. The Jupiter mass is a unit of mass equal to the total mass of planet Jupiter
- The center of a Galaxy does not contain a Giant Star it contains a Supermassive Black Hole.
- A Red Dwarf is not a Dwarf Planet it is a Star. Most common Stars are Red Dwarf (cool Stars like Tarf)
Commonly Asked Questions
Q. What prevents us seeing the Stars in the night sky?
A. Light pollution, fog, city lights and artificial lights all limit our visibility of the objects in the sky at night.
Q. Will the locations of Stars change over time?
A. Stars are continually on the move.
The images we form in our imagination to make objects, shapes and patterns out of the constellations have already shifted over time.
As we view the night skies from Earth they are likely to continue to shift and possibly in time the images may look very different.
- Tarf image – https://theskylive.com/sky/stars/tarf-beta-cancri-star
- Nebula – By Credit: NASA, Jeff Hester, and Paul Scowen (Arizona State University) – http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/34/image/a, Public Domain, https://commons.wikimedia.org/w/index.php?curid=129538
- Supernova Remnant – Cassiopeia A. Credit: NASA/CXC/SAO – https://scitechdaily.com/lonely-origin-of-cassiopeia-a-revealed-one-of-the-most-famous-supernova-remnants
- Milky way – Source: Nick Risinger – https://sedsvit.medium.com/just-us-d4ab577099a2
- Celestial Pole – By I, Dennis Nilsson, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=3262268
- Tarf color – https://vignette.wikia.nocookie.net/startrek/images/a/a4/Beta_Lankal_primary.jpg/revision/latest/top-crop/width/360/height/450?cb=20140118211513
- Johann Bayer – https://laexuberanciadehades.files.wordpress.com/2012/10/johann-bayer.jpg