Human Reproductive System

Male Reproductive System

The male reproductive system is illustrated to the right. Sperm are produced in the testes located in the scrotum. Normal body temperature is too hot thus is lethal to sperm so the testes are outside of the abdominal cavity where the temperature is about 2° C (3.6° F) lower. Note also that a woman’s body temperature is lowest around the time of ovulation to help insure sperm live longer to reach the egg. If a man takes too many long, very hot baths, this can reduce his sperm count. Undescended testes (testes are supposed to descend before birth) will cause sterility because their environment is too warm for sperm viability unless the problem can be surgically corrected. (Undescended testes are also very prone to developing cancer, thus if they cannot be surgically moved, they probably will need to be removed.)

From there, sperm are transferred to the epididymis, coiled tubules also found within the scrotum, that store sperm and are the site of their final maturation.

In ejaculation, sperm are forced up into the vas deferens (plural = vasa deferentia). From the epididymis, the vas deferens goes up, around the front of, over the top of, and behind the bladder. A vasectomy is a fairly simple, outpatient operation that involves making a small slit in each scrotum, cutting the vasa deferentia near where they begin, and tying off the cut ends to prevent sperm from leaving the scrotum. Because this is a relatively non-invasive procedure (as compared to doing the same to a woman’s oviducts), this is a popular method of permanent birth control once a couple has had all the children they desire. Couples should carefully weigh their options, because this (and the corresponding female procedure) is not designed to be a reversible operation.

The ends of the vasa deferentia, behind and slightly under the bladder, are called the ejaculatory ducts. The seminal vesicles are also located behind the bladder. Their secretions are about 60% of the total volume of the semen (= sperm and associated fluid) and contain mucus, amino acids, fructose as the main energy source for the sperm, and prostaglandins to stimulate female uterine contractions to move the semen up into the uterus. The seminal vesicles empty into the ejaculatory ducts. The ejaculatory ducts then empty into the urethra (which, in males, also empties the urinary bladder).

The initial segment of the urethra is surrounded by the prostate gland (note spelling!). The prostate is the largest of the accessory glands and puts its secretions directly into the urethra. These secretions are alkaline to buffer any residual urine, which tends to be acidic, and the acidity of the woman’s vagina. The prostate needs a lot of zinc to function properly, and insufficient dietary zinc (as well as other causes) can lead to enlargement which potentially can constrict the urethra to the point of interferring with urination. Mild cases of prostate hypertrophy can often be treated by adding supplemental zinc to the man’s diet, but severe cases require surgical removal of portions of the prostate. This surgery, if not done very carefully can lead to problems with urinary incontinence or sexual performance.

The bulbourethral glands or Cowper’s glands are the third of the accessory structures. These are a small pair of glands along the urethra below the prostate. Their fluid is secreted just before emission of the semen, thus it is thought that this fluid may serve as a lubricant for inserting the penis into the vagina, but because the volume of these secretions is very small, people are not totally sure of this function.

The urethra goes through the penis. In humans, the penis contains three cylinders of spongy, erectile tissue. During arousal, these become filled with blood from the arteries that supply them and the pressure seals off the veins that drain these areas causing an erection, which is necessary for insertion of the penis into the woman’s vagina. In a number of other animals, the penis also has a bone, the baculum, which helps to stiffen it. The head of the penis, the glans penis, is very sensitive to stimulation. In humans, as in other mammals, the glans is covered by the foreskin or prepuce, which may have been removed by circumcision. Medically, circumcision is not a necessity, but rather a cultural “tradition”. Males who have not been circumcised need to keep the area between the glans and the prepuce clean so bacteria and/or yeasts don’t start to grow on accumulated secretions, etc. there. There is some evidence that uncircumcised males who do not keep the glans/prepuce area clean are slightly more prone to penile cancer.

Female Reproductive System

The female reproductive system is illustrated to the right. “Eggs” are produced in the ovaries, but remember from our discussion of meiosis, that these are not true eggs, yet, and will never complete meiosis and become such unless/until first fertilized by a sperm. Within the ovary, a follicle consists of one precursor egg cell surrounded by special cells to nourish and protect it. A human female typically has about 400,000 follicles/potential eggs, all formed before birth. Only several hundred of these “eggs” will actually ever be released during her reproductive years. (As just an example, if a woman would ovulate from age 15 to age 55, which is a long time, that’s 40 years. If we’d assume 13 28-day cycles per year, that would be 40 × 13 = 520 potential times an “egg” could be released.) Normally, in humans, after the onset of puberty, due to the stimulation of follicle-stimulating hormone (FSH) one “egg” per cycle matures and is released from its ovary. Ovulation is the release of a mature “egg” due to the stimulation of leutenizing hormone (LH), which then stimulates the remaining follicle cells to turn into a corpus luteum which then secretes progesterone to prepare the uterus for possible implantation. If an egg is not fertilized and does not implant, the corpus luteum disintegrates and when it stops producing progesterone, the lining of the uterus breaks down and is shed.

Each “egg” is released into the abdominal cavity near the opening of one of the oviducts or Fallopian tubes. Cilia in the oviduct set up currents that draw the egg in. If sperm are present in the oviduct (if the couple has recently had intercourse), the egg will be fertilized near the far end of the Fallopian tube, will quickly finish meiosis, and the embryo will start to divide and grow as it travels to the uterus. The trip down the Fallopian tube takes about a week as the cilia in the tube propel the unfertilized “egg” or the embryo down to the uterus. At this point, if she had intercourse near the time of ovulation, the woman has no idea whether an unfertilized “egg” or a new baby is travelling down that tube. During this time, progesterone secreted by the corpus luteum has been stimulating the endometrium, the lining of the uterus, to thicken in preparation for possible implantation, and when a growing embryo finally reaches the uterus, it will implant in this nutritious environment and begin to secrete its own hormones to maintain the endometrium. If the “egg” was not fertilized, it dies and disintegrates, and as the corpus luteum also disintegrates, its progesterone production falls, and the unneeded, built-up endometrium is shed.

The uterus has thick, muscular walls and is very small. In a nulliparous woman, the uterus is only about 7 cm long by 4 to 5 cm wide, but it can expand to hold a 4 kg baby. The lining of the uterus is called the endometrium, and has a rich capillary supply to bring food to any embryo that might implant there.

The bottom end of the uterus is called the cervix. The cervix secretes mucus, the consistency of which varies with the stages in her menstrual cycle. At ovulation, this cervical mucus is clear, runny, and conducive to sperm. Post-ovulation, the mucus gets thick and pasty to block sperm. Enough of this mucus is produced that it is possible for a woman to touch a finger to the opening of her vagina and obtain some of it. If she does this on a daily basis, she can use the information thus gained, along with daily temperature records, to tell where in her cycle she is. If a woman becomes pregnant, the cervical mucus forms a plug to seal off the uterus and protect the developing baby, and any medical procedure which involves removal of that plug carries the risk of introducing pathogens into the nearly-sterile uterine environment.

The vagina is a relatively-thin-walled chamber. It servs as a repository for sperm (it is where the penis is inserted), and also serves as the birth canal. Note that, unlike the male, the female has separate opening for the urinary tract and reproductive system. These openings are covered externally by two sets of skin folds. The thinner, inner folds are the labia minora and the thicker, outer ones are the labia majora. The labia minora contain erectile tissue like that in the penis, thus change shape when the woman is sexually aroused. The opening around the genital area is called the vestibule. There is a membrane called the hymen that partially covers the opening of the vagina. This is torn by the woman’s first sexual intercourse (or sometimes other causes like injury or some kinds of vigorous physical activity). In women, the openings of the vagina and urethra are susceptible to bacterial infections if fecal bacteria are wiped towards them. Thus, while parents who are toilet-training a toddler usually wipe her from back to front, thus “imprinting” that sensation as feeling “right” to her, it is important, rather, that that little girls be taught to wipe themselves from the front to the back to help prevent vaginal and bladder infections. Older girls and women who were taught the wrong way need to make a conscious effort to change their habits.

At the anterior end of the labia, under the pubic bone, is the clitoris, the female equivalent of the penis. This small structure contains erectile tissue and many nerve endings in a sensitive glans within a prepuce which totally encloses the glans. This is the most sensitive point for female sexual stimulation, so sensitive that vigorous, direct stimulation does not feel good. It is better for the man to gently stimulate near the clitoris rather than right on it. Some cultures do a procedure, similar to circumcision, as a puberty rite in teenage girls in which the prepuce is cut, exposing the extremely-sensitive clitoris. There are some interesting speculations on the cultural significance of this because the sensitivity of the exposed clitoris would probably make having sexual intercourse a much less pleasant experience for these women.

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Type of Fertilization

Reproductive System

Animals’ reproductive systems can be divided into the internal reproductive organs and the external genitalia. The gonads are the actual organs that produce the gametes. In the male, testes (singular = testis) produce sperm, and in the female, ovaries make eggs.

In most animals, individuals are either definite males or definite females. However, in some species, individual organisms are both male and female. Hermaphroditism is when one organism has both sexes. Earthworms and garden snails always have both male and female organs, and when, for example, two earthworms mate, they fertilize each other. A special variation on the theme is sequential hermaphroditism, in which an organism changes sex during its life. If an organism is female first and later changes to male, that organism is protogynous, and if the organism is male first and changes to female, it is said to be protandrous. In different species, sequential hermaphroditism can be influenced by the organism’s age or size or by various environmental/climatic factors.

While most higher animals reproduce sexually, there are some species in which the females can, under certain conditions, produce offspring without mating. Parthenogenesis is the ability of an unfertilized egg to develop and hatch. This seems to be especially prevalent among insects. Some of the giant walkingsticks at the Zoo are females who, without mating, lay eggs that hatch into more females generation after generation. Other insects, like some aphids, have complicated life cycles that involve sexually-reproducing generations alternating with parthenogenically produced generations. In honeybees, fertilized eggs turn into females (workers and queens), while unfertilized eggs, which are only produced in the spring, turn into males.

In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm and eggs are designed to be in a watery environment, aquatic animals can make use of the water in which they live, but terrestrial animals must, in some way, provide the wet environment needed for the sperm to swim to the egg. There are, thus, two major mechanisms of fertilization. In external fertilization, used by many aquatic invertebrates, eggs and sperm are simultaneously shed into the water, and the sperm swim through the water to fertilze the egg. In internal fertilization, the eggs are fertilized within the reproductive tract of the female, and then are covered with eggshells and/or remain within the body of the female during their development.

In species with external fertilization, at an appropriate developmental stage, the eggs hatch, and the new young simply swim away. However, females of species with internal fertilization must, at some point, expel the growing young. There are three general ways of doing this:

·      Oviparous organisms, like chickens and turtles, lay eggs that continue to develop after being laid, and hatch later.

·        Viviparous organisms, like humans and kangaroos, are live-bearing. The developing young spend proportionately more time within the female’s reproductive tract, portions of which are specially-modified for this purpose. Young are later released to survive on their own.

·         Ovoviviparous organisms, like guppies, garter snakes, and Madagascar hissing roaches, have eggs (with shells) that hatch as they are laid, making it look like “live birth.” 

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Sun

What is the Sun?

The Sun is what is known as a main sequence star; that is, a sphere composed primarily of the two gases hydrogen and helium such that certain conditions are met. The first condition is that it must have a mass falling within a certain range. Though debated, this range is generally accepted to be between approximately 1.4 x 1029 kg and 3.0 x 1032 kg. (This range is often describe as at least 75 times the mass of Jupiter and no more than 150 times the mass of the Sun itself.) The second and most important condition is that nuclear fusion must be present. Nuclear fusion is the process whereby two lighter atomic nuclei join or "fuse" together to produce a heavier atomic nucleus. In the context of stars, hydrogen is the lighter and helium the heavier.

What Type of Star is the Sun?

Although we think of our Sun as a unique celestial body, it is, in fact, one of trillions of stars in the universe. On top of this, the Sun is rather ordinary as far as stars go. The official classification for our Sun is G V star (often refered to as a Yellow Dwarf star), which means that it is a main sequence star whose surface temperature is between 5027°C and 5727°C.

Some estimates for stars similar to the Sun in the Milky Way galaxy alone are as high as 7 billion. If this number is correct, there could be over one trillion stars that are roughly the same as our Sun in the universe.

Does the Sun Have Another Name?

While our Sun does not have an official scientific name, it does have another common name: Sol. This name originates from the ancient Roman's god of the Sun, Sol. This alternate name is where we get the term "solar system," which literally means system of the Sun.

Size of the Sun

The size of the Sun compared to the largest known stars (red giants) is not very big. However, if compared to the most common type of star in the universe, the red dwarf, the Sun is quite a bit larger. Thus, the Sun is not the biggest type of star in the universe, but it is definitely larger than most.

As far as the Sun's mass compared to other bodies found in our solar system, the Sun is easily the most massive. The Sun alone contains 99.8% of the total mass in the Solar System. In terms of size, the Sun has a diameter of roughly 1.4 million kilometers (870,000 miles). To put this in perspective, this is almost 110 times the diameter of the Earth. What this means is that about one million Earth's could fit inside the Sun.

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Planets

What is a Planet?

The answer to this question is a highly controversial one. This has not always been the case, though. In fact, before 1978 the definition of a "planet" was not really necessary. Until that time a planet simply meant a body in orbit around the Sun, that reflected sunlight, and was not a planetary moon, asteroid, or comet.

However, with the discovery of Pluto's moon Charon in 1978 scientists were able to calculate Pluto's mass much more accurately than ever before and soon realized that it was much smaller than they had previously believed. At a tiny fraction of the mass of Mercury, Pluto was clearly a body much smaller than any other planet. This discovery led some to question whether Pluto was actually a planet or some other type of object.

In the 1990s and early 2000s the discovery of several objects in the outer solar system similar in size to Pluto made it all but necessary to come to a definitive definition of a planet. Such a definition was needed to separate those types of objects like Pluto into a distinct class, otherwise all of the newly found objects would have to be called planets as well.

In response to this uncertainty, the International Astronomical Union (IAU), the official governing body for matters concerning naming astronomical objects, came to a definition of the term "planet." According to the IAU, a planet is a celestial body that meets the following criteria: is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit.

How Many Planets are in the Solar System?

According to the IAU's definition for planet above, there are 8 known planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer considered a planet under the IAU definition.

Are There Planets in the Solar System that Have Not Been Discovered?

There have been several additional planets hypothesized throughout history; however, none of these planets has ever been found.

The most recent of these theorized planets was Planet X, a supposedly giant planet used to explain the deviations from the predicted orbits of Uranus and Neptune. Although Planet X has never been found and scientists are still unable to satisfactorily explain the uranian and neptunian orbital discrepancies, the scientific community has almost unanimously come to the conclusion that Planet X does not exist. It is highly unlikely that there are any planets beyond the orbit of Pluto.

Types of Planets

The planets fall into two categories based on their physical characteristics:

·            The terrestrial planets

There are four terrestrial planets: Mercury, Venus, Earth, and Mars. These planets are those closest to the Sun. They are characterized by their dense, rocky composition.

·            The gas giants.

There are four gas giant planets: Jupiter, Saturn, Uranus, Neptune. These planets are found in the outer solar system. They are characterized by their massiveness and gaseous composition.

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Vision Defects

An image is focused exactly on the retina for a person with normal vision. The common defects of vision are : short-sightedness, long-sightedness, astigmatism, colour blindness and presbyopia. 

Long-sightedness (Myopia)

Light ray which enter a normal eye will be focus on the retina.

When someone suffer from long-sightedness has the condition of unable to see the near objects clearly. The two causes: the eye lens is too thin or the eyeball is too short. Therefore, light from distant object is focused at the back of the retina (as shown in the diagram above). The correction method is to wear glasses with convex lenses. The lenses converge light rays before they enter the eye to form a sharp image on the retina.

  Short-sightedness (Myopia)

Can not view clearly for far object, the light focus in front of the retina

 

When someone suffer from short-sightedness has the condition of unable to see the distant objects clearly. The two causes: the eye lens is too tick and the eyeball is too long. Therefore, light from distant object is focused in front of the retina (as shown in the diagram above). The correction method is to wear glasses with concave lenses. The lenses diverge light rays before they enter the eye to form a sharp image on the retina.

 Colour Blindness

Due to shortage of cone cells on the retina. Most of the people who suffer from colour blindness can not differentiate between red and green colours. The situation can not be corrected because it is hereditary. People with colour blindness can not read "8" from the picture, because they can not differentiate green dots and red dots.

Astigmatism

 Astigmatism is caused by the irregular surface of the cornea or lens or both. The eye is unable to focus objects into a single, sharp image. Therefore, the images seen by astigmatic patient are distorted and not clear. To correct the situation, the patient have to wear glasses with cylindrical lenses, wear special contact lenses or undergoes surgery.

Limitations of sight

We can not see microscopic objects, distant objects, we can not see through opaque objects and when the light fall on the blind spot.

OPTICAL ILLUSION

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Properties of Light

Light is a form of energy that travels at the speed 300,000,000 m/s. Approximately 7.5 rounds of the earth in 1 second. Light travels in a straight lines within the same medium (or medium with the same optical density). Light is reflected when it hits an opaque surface. Light is reflected when it travels from one transparent medium to another.

REFLECTION OF LIGHT

Light which falls on an opaque object may be absorb or reflected. Reflection takes place when light rays which fall on the surface of an opaque object bounces off it. The amount and direction of the reflected light depend on the type of surface the light fall on.

Incident ray is the ray of light that approaches the surface. Reflected ray is the ray of light that bounces off the surface of an object.

Rough and uneven surfaces such as cloth and wood (right) cause diffused reflection (light reflection is scattered.

Smooth and flat surfaces such as mirrors, glass and shinny metals (left) cause regular reflection (light reflected regularly).

Application of reflection of light in daily life

Plane mirrors are used in bathrooms or bedrooms. Side mirrors and rear-view mirrors are used in cars. Convex mirrors give wider scope of view and used as security mirrors in shop and fish-eye mirrors at road bends. Concave mirrors magnify images and are  used in microscopes and telescopes. Periscopes are used to see above the water surface in submarines. A periscope consists of two plane mirror and change the direction of light. Kaleidoscopes form colourful images. They are made from two or more plane mirrors which reflect the light from objects placed between the mirrors.

1.      Convex mirror used in a supermarket

2.      Kaleidoscopes for kids

3.      View of pattern inside kaleidoscopes

REFRACTION OF LIGHT

Light can travel through transparent media such as air, water and glass. Light travels at different speeds in different media. When light travels from one medium into another, the change in the speed of light causes the light to bend or change direction. Therefraction of light is the bending of light as light travels through two types of media that have different densities (optical density).The speed of light increases when light enters a less dense medium meanwhile the speed of light decreases when light enters a denser medium. Refraction of light does not occur when the light ray is directly perpendicular to the surface of the medium (the light travel into the medium in the direction of the normal line), the light will pass through the medium in a straight line.

The refraction of light ray 

 Phenomenon of refraction of light. The pencil which is in water appear bended to the observer.

   

  Phenomenon of refraction of light. Scuba diver appear closer to the observer when it is in the water. 

  

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Physical and Chemical Changes

Physical changes are limited to changes that result in a difference in display without changing the composition. Some common changes (but not limited to) are: 

·         Texture

·         Color

·         Temperature

·         Shape

Change of State (Boiling Point and Melting Point are significant factors in determining this change.)

Physical properties include many other aspects of a substance. The following are (but not limited to) physical properties. 

·  Luster

·  Malleability

·    Ability to be drawn into a thin wire

·         Density

·         Viscosity

·         Solubility

·         Mass

·         Volume 

Any change in these physical properties is referred to as a physical change. For further information, please refer to Properties of Matter.

Chemical changes, on the other hand, are quite different. A chemical change occurs when the substance's composition is changed. When bonds are broken and new ones are formed a chemical change occurs. The following are indicators of chemical changes:

·         Change in Temperature

·         Change in Color

·         Noticeable Odor (after reaction has begun)

·         Formation of a Precipitate

·         Formation of Bubbles

Common Physical Changes

Texture

The texture of a substance can differ with a physical change. For example, if a piece of wood was sanded, waxed, and polished, it would have a very different texture than it initially had as a rough piece of wood.

Rough Wood                                                          Finished Wood

    

Color

The changing of color of a substance is not necessarily an indicator of a chemical change. For example, changing the color of a metal does not change its physical properties. However, in a chemical reaction, a color change is usually an indicator that a reaction is occurring. The guy painting the metal car is not changing the composition of the metallic substance.

Painting a Car

Temperature

Although we cannot see temperature change, unless if a change of state is occurring, it is a physical change.

 

One cannot see the pan physically changing shape, color, texture, or any of the other physical properties. However, if one were to touch the pan, it would be incredibly hot and could cause a burn. Sitting idle in a cupboard, this pan would be cold. One cannot assess this change only through visual exposure; the use of a thermometer or other instrument is necessary.

Shape

The shape of an object can be changed and the object will still remain true to its chemical composition. For example, if one were to fold money, as shown by the figure below, the money is still chemically the same.

To more understand about materials changes you can see this ppt:

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Solar System Formation

The solar system is a group of celestial bodies are made up of the sun as the center and source of light surrounded by planets and their satellites, asteroids (planetoid), comets, and meteors.

Theory of Solar System Formation

Teori Kabut

Imanuel Kant (1724-1804), berkebangsaan Jerman, membuat suatu teori tentang terjadinya tata surya. Dikatakan bahwa di jagad raya terdapat gunpalan kabut yang berputar perlahan-lahan. Bagian tengah kabut itu lama-kelamaan menjadi gumpalan gas yang kemudian menjadi matahari dan bagian kabut sekitarnya menjadi planet-planet dan satelitnya.

Teori Planetesimal

Thomas C . Chamberlin & Forest R Moulton seorang ahli astronomi mengemukakan teori yang dikenal dengan teori planetesimal yang berarti planet kecil. Teori ini menyatakan bahwa matahari telah ada sebagai salah satu dari bintang. Suatu saat matahari berpapasan dengan sebuah bintang dengan jarak yang tidak terlalu jauh sehingga terjadi peristiwa pasang naik pada permukaan matahari maupun bintang itu, serta bagian dari massa matahari tertarik kearah bintang. Pada waktu bintang tersebut menjauh, sebagian dari massa matahari jatuh kembali ke permukaan matahari dan sebagian lagi terhambur keluar angkasa disekitar matahari. Hal inilah yang dinamakan planetesimal yang kemudian menjadi planet-planet yang beredar mengelilingi orbitnya.

Teori Pasang Surut Bintang

Teori pasang surut bintang pertama kali dikemukakan oleh James Jeans pada tahun 1917. Planet dianggap terbentuk karena mendekatnya bintang lain kepada matahari. Keadaan yang hampir bertabrakan menyebabkan tertariknya sejumlah besar materi dari matahari dan bintang lain tersebut oleh gaya pasang surut bersama mereka, yang kemudian terkondensasi menjadi planet, Namun astronom Harold Jeffreys tahun 1929 membantah bahwa tabrakan yang sedemikian itu hampir tidak mungkin terjadi. Demikian pula astronom Henry Norris Russell mengemukakan keberatannya atas hipotesis tersebut.

Teori Vorteks

Dikemukakan oleh Karl Von Weiszackermenurut Weiszacker, nebula (kabut) terdiri atas vorteks-vorteks (pusaran-pusaran) yang merupakan sifat gerakan gas. Gerakan gas dalam nebula menyebabkan pola sel-sel yang bergolak (turbulen). Pada batas antar sel turbulen, terjadi tumbukan antar partikel yang kemudian membesar dan menjadi planet.

Hypothesis Of Solar System’ Formation

1.    Nebula

Menurut Hipotesis ini, planet berasal dari kabut pijar yang berputar membentuk gelang-gelang, berbentuk Gumpalan kemudian membeku menjadi Planet. Teori ini disampaikan oleh Immanuel Kant dan Piere Simon de Laplace. 

a.       Nebula berasal dari gas dan debu, sebagian besar menjadi Matahari.

b.      Terbentuk Matahari dan planet lain yang masih Berpijar.

c.       Matahari terbentuk planet-planet bertebaran tak terarah.

d.      Matahari berputar pada porosnya, planet-planet terbentuk atmosfernya.

e.       Planet terbentuk atmosfer, dibumi telah muncul kehidupan karena sudah ada lapisan atmosfer.

2.    Pasang Surut Bintang

Menurut Hipotesis ini, adanya gaya tarik menarik antara matahari dengan bintang besar, menyebabkan terbentuknya tonjolan seperti serutu pada matahari. Serutu itu lepas dan terputus-putus yang membentuk tetesan-tetesan yang memadat sehingga terbentuk planet. Teori ini dikemukakan oleh James Jeans. Namun, gas darimanakah yang terlepas sehingga terbentuk planet? Perlu  dilakukan penyelidikan lebih lanjut.

3.    Planetisimal

Menurut Thomas C. Chamberlain dan Forest R. Moulton pada tahun 1900, matahari dan bintang lainnya pada suatu saat melintas sangat dekat satu sama lain. Akibatnya, terjadilah semacam pasang dan gas yang besar disedot dari matahari oleh tarikan gravitasi bintang lain tersebut. Karena adanya pasang surut ini maka gas-gas tersebut berputar mengelilingi matahari lalu mulai mengalami penurunan suhu dan memadat membentuk partikel-partikel keras dalam ukuran yang berbeda yang disebut planetasimal (planet kecil). Partikel-partikel yang lebih besar yang disebut Knot, bertindak sebagai inti untuk pembentukan planet-planet itu. Inti tersebut menarik dan bergabung dengan planet-planetesiamal lainnya yang lebih kecil dan akhirnya menjadi massa yang lebih besar sehingga membentuk planet-planet yang lebih kecil itu menjadi planetoida, meteor bahkan satelit-satelit dari planet-planet.

4.    Kondensasi

Hipotesis kondensasi mulanya dikemukakan oleh astronom Belanda yang bernama G.P. Kuiper (1905-1973) pada tahun 1950. Hipotesis kondensasi menjelaskan bahwa tata surya terbentuk dari bola kabut raksasa yang berputar membentuk cakram raksasa.

5.      Bintang Kembar
Hipotesis bintang kembar awalnya dikemukakan oleh Fred Hoyle (1915-2001) pada tahun 1956. Hipotesis mengemukakan bahwa dahulunya tata surya kita berupa dua bintang yang hampir sama ukurannya dan berdekatan yang salah satunya meledak meninggalkan serpihan-serpihan kecil.

6.      BigBang
Terbentuknya alam semesta dan tata surya diawali dari dentuman yang dahsyat meledak, menyebarlah serpihan debu dan awan hidrogen, hasil ledakan berupa debu dan awan hidrogen membentuk bintang-bintang. Matahari merupakan salah satunya. Akibat adanya gaya gravitasi antarmolekul menyebabkan terjadinya gerakan memutar, bagian pusat menjadi Matahari, sedangkan gumpalan lainnya menjadi planet-planet. Ketika daya pancar sinar matahari semakin besar, selubung gas yang letaknya lebih dekat dengan matahari tersapu sehingga ukurannya menjadi lebih kecil dan padat. Planet yang atmosfernya tersapu bersih adalah merkurius dan venus, sedangkan bumi merupakan planet ketiga yang berjarak ideal.

Untuk mempelajari materi tata surya lebih lanjut kamu dapat melihat ppt berikut:

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