1. There was rapid change is brightness seen. The brightness increased some 400 times which is huge as most comets that pass by the sun only brighten up by 10 to 100 times.
2. Light emitted by 3I Atlas was close to blue where normal expectation was it will be red due to coma of gas and dust.
3.  3I Atlas started to experience non gravitational acceleration for the first time during its perihelion with the Sun. This is sort of expected as coma of the comet push around the object giving it non gravitational acceleration. The magnitude however is higher than expected.
4. Its closest approach to the Sun was on October 29^th 2025
5. Data taken after closest approach shows seven distinct jets of gas going into space and are mostly pointing away from the sun. Couple of smaller jets are shooting directly into the sun which is not typical.
6. Actual size of 3I Atlas is still debatable but its roughly estimated between 5km to 10km across and its more likely to be on the small side of the estimation range. This make 3I Atlas 20 times larger than Omua Mua.  

Let M be the mass of the planet and m be the mass of its satellite. For the satellite to be in the circular orbit, gravitational force is equal to centripetal force.

If r be the radius of the orbit of the satellite,

If T is the period of revolution of the satellite,

Then,

So, knowing the distance of the satellite from the planet and time period of revolution of the satellite, the mass of the planet can be determined.

If two stars of magnitude and possess brightness and respectively, then

This was a simple definition but there are few other definition that is used less often.

Tropical year: Tropical year is the time taken for the tilt of the earth’s axis to come back to the same angle relative to the sun. (365.242 days)

Sidereal year: Sidereal year is the time amount of time before we come back and see the same stars rising behind the sun. (365.256 days)

Anomalistic year: We know that earth precesses while following its elliptical orbit and the amount of time between closest approaches to the sun is called anomalistic year. (365.260 days)

The time length will vary in all three types of years slightly. We use tropical year in our calendar because the use of this year makes it easy to mark the passage of the seasons.

The duration of year of every planet in the solar system vary. For example Mercury’s year is roughly 88 earth days and Neptune’s year is 165 earth years. There is also a galactic year and it is the amount of time solar system takes to orbit around Milky Way (about 0.25 billion earth years).

Leap Year

As we have already see a year has about 365 days plus some change. We will talk about the change later but even 365 cannot be divided nicely into 12 equal parts so our months have days ranging from 28 to 31 days. In the month of February there are generally 28 days but due to the change I have mentioned earlier, there is a month of February with 29 days in it. This happens every 4 years and this year when February has 29 days is called leap year.

So in a leap year there are 366 days. We are so used to relating season with months and the lack of leap year adjustment will slowly make our calendar go out of sync. Without the adjustment, every passing year will slowly increases the sync problem and in 30 years the calendar will be off by a week and in few hundred years the seasons would be flipped meaning we will be celebrating Christmas in summer.

How can this happen? As it turns out a year is about 365 days and six hours. So every year this six hours pile up making our calendar go out of sync. Apparently 365 days a year is too few and making a year 366 days is too much which again will disrupt the sync. So, the solution that came up was a leap year when we add an extra day on February after every four years.

Problem solved …. Right? No …

As it happens adding a day every four year is a little too much and it will disrupt our sync once again by one day per hundred years. This is not bad but to achieve more accuracy, something new is done. To fix this problem, a leap year is skipped every century. For example 1896 and 1904 was a leap year but 1900 wasn’t. Even this adjustment will have an error and our calendar will go faster by one day in 400 years. To solve this problem another condition is added. This rule says that if a century is divisible by 400 than it will be a leap year. So, to sum it up 1900 and 2100 aren’t leap years but 2000 was a leap year.

With this last adjustment the calendar is more accurate and error is about one day off in almost eight thousand years. This is good enough for now and if it comes to adjusting calendar after eight thousand year, we will see to it then. For now we are good.

Drake equation contrary to many other equation we have seen in Physics have too many unknowns so the answers coming out of it has some uncertainties. The variables in Drake equation are based on our best guess or estimation. So, as we increase our knowledge of the universe, the result coming out from Drake’s equation becomes more certain and dependable. The most definite answer to Drake equation can only be achieved only if institution like SETI succeeds in finding other intelligent species in vastness of the universe. The Drake equation tries to find the number of technological civilizations in the Milky way galaxy by using the equation

All these factors multiplied together helps us to find the intelligent civilizations that we might detect right now.

is the rate of formation of stars in the Milky way galaxy over the last few billion years and hence it is the number of stars per year. Since Milky Way is about 10 billion years old, in earlier time stars’ rate of formation was different. All the f factors in Drake equation are fractions hence each one is less or equal to 1.
is the fraction of stars having planets.
is the average number of habitable planets in a solar system
is the fraction of planets where life actually starts
is the fraction of the life which develops intelligent life
is the fraction of that intelligent life developing a civilization and uses some sort of transmitting technology
L is what we called as longevity factor or the factor deciding the transmitters’ continuous operation.

The first three factors are easier to predict as we are finding more exoplanets everywhere. The product of first three terms are more certain at this point. The more difficult factor is the fractions dealing with life and intelligence and technological civilizations. At this point the only known planet harboring intelligent life is right here on Earth. As we explore Mars, Europa and Titan, any kind of life detection in these places means that life will be abundant in the Milky Way because if life can start twice in one solar system than in suitable condition life will happen.

The unknown and uncertainties are great at this point but we think that are both numbers close to 10 and f factors are less than one and some way less than one. Among the unknowns, the biggest one in L. Unless L is large, the chance of N being large is slim. But on the other hand if we detect sign of intelligent life in small portion of sky then we can extrapolate that L is large in the bigger scale.

Since the speed of light is finite, any signal for intelligent life we receive will tell us about the past of that civilization and not what happening now. The biggest source of predicting the value of L is humans itself. In our very short existence in cosmic scale, we have developed technologies to go to the moon and on the other hand we have also developed weapon of mass destruction, chemical weapon, biological weapon etc. Will this development help us or hinder us in long lifetime will give an idea to fate of other civilization as well.

At this point Drake equation have a lot of unknown and there is no right answer to this equation so far.

The concept of a body so massive that even light could not escape was put forward by the geologist John Michell in a letter written to Henry Cavendish in 1783.

“If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.”

This assumes that light is influenced by gravity in the same way as massive objects.

The idea of black holes was largely ignored in the nineteenth century, since light was then thought to be a massless wave and therefore not influenced by gravity.

Definition

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a “singularity “. As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.

A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.

Size of Black Holes

There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. If we think about the masses of black holes, there is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if we compress it to a high enough density. We suppose that most of the black holes that actually exist out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star.

Black holes can be divided into several size categories:

• Super massive black holes that contain millions to billions of times the mass of the sun are believed to exist in the center of most galaxies, including our own Milky Way. They are thought to be responsible for active galactic nuclei.
• Intermediate-mass black holes, whose size is measured in thousands of solar masses, may exist. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources.
• Stellar-mass black holes have masses ranging from about 1.5-3.0 solar masses (the Tolman-Oppenheimer-Volkoff limit) to 15 solar masses. These black holes are created by the collapse of individual stars. Stars above about 20 solar masses may collapse to form black holes; the cores of lighter stars form neutron stars or white dwarf stars. In all cases some of the star’s material is lost (blown away during the red giant stage for stars that turn into white dwarfs, or lost in a supernova explosion for stars that turn into neutron stars or black holes).
• Micro black holes, which have masses at which the effects of quantum mechanics are expected to become very important. This is usually assumed to be near the Planck mass. Alternatively, the term micro black hole or mini black hole may refer to any black hole with mass much less than that of a star. Black holes of this type have been proposed to have formed during the Big Bang (primordial black holes), but no such holes have been detected as of 2007.

Types of Black Holes

Astrophysicists currently classify black holes according to their angular momentum (non-zero angular momentum means the black hole is rotating) and electric charge:

Astrophysicists expect that almost all black holes will rotate, because the stars from which they are formed rotate. In fact most black holes are expected to spin very rapidly, because they retain most of the angular momentum of the stars from which they were formed but concentrated into a much smaller radius.

Major features of non-rotating, uncharged black holes

Event horizon  

This is the boundary of the region from which not even light can escape, but at the same time, light does not get sucked into the black hole. Stephen Hawking, in his book, “A Brief History of Time”, describes the event horizon as “the point at which light is just barely unable to escape.” The event horizon is the defining feature of a black hole – it is black because no light or other radiation can escape from inside it. So the event horizon hides whatever happens inside it and we can only calculate what happens by using the best theory available, which at present is general relativity. The gravitational field outside the event horizon is identical to the field produced by any other spherically symmetric object of the same mass. The popular conception of black holes as “sucking” things in is false. Objects can maintain an orbit around black holes indefinitely provided they stay outside the photon sphere.

Singularity

According to general relativity, a black hole’s mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time which it causes. These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a “singularity”.

A photon sphere

A non-rotating black hole’s photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times larger than the radius of the event horizon.

Accretion disk

Space is not a pure vacuum even interstellar space contains a few atoms of Hydrogen per cubic centimeter. The powerful gravity field of a black hole pulls this towards and then into the black hole. The gas nearest the event horizon forms a disk and, at this short range, the black hole’s gravity is strong enough to compress the gas to a relatively high density. The pressure, friction and other mechanisms within the disk generate enormous energy. Infect they convert matter to energy more efficiently than the nuclear fusion processes that power stars. As a result, the disk glows very brightly, although disks around black holes radiate mainly X-rays rather than visible light. However Accretion disks are not proof of the presence of black holes, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks to form and to behave in the same ways as those around black holes.

Major features of rotating black holes

Rotating black holes share many of the features of non-rotating black holes – inability of light or anything else to escape from within their event horizons, accretion disks, etc. But general relativity predicts that rapid rotation of a large mass produces further distortions of space-time in addition to those which a non-rotating large mass produces, and these additional effects make rotating black holes strikingly different from non-rotating ones.

Two important surfaces around a rotating black hole. The inner sphere is the static limit (the event horizon). It is the inner boundary of a region called the ergosphere. The oval-shaped surface, touching the event horizon at the poles, is the outer boundary of the ergosphere. Within the ergosphere a particle is forced to rotate and may gain energy at the cost of the rotational energy of the black hole.

What happens if something falls into a black hole?

An object in very strong gravitational field feels a tidal force stretching it in the direction of the object generating the gravitational field. This is because of the inverse square law. It causes nearer parts of the stretched object to feel a stronger attraction than farther parts. Near black holes, the tidal force is expected to be strong enough to deform any object falling into it, even atoms or composite nucleons, this is called spaghettification.

An object in a gravitational field experiences a slowing down of time, called gravitational time dilation, relative to observers outside the field. The observer will see that physical processes in the object, including clocks, appear to run slowly. As a test object approaches the event horizon, its gravitational time dilation (as measured by an observer far from the hole) would approach infinity.

From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon: and it appears to become redder and dimmer, because of the extreme gravitational red shift caused by the gravity of the black hole. Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon. All of this is a consequence of time dilation: the object’s movement is one of the processes that appear to run slower and slower, and the time dilation effect is more significant than the acceleration due to gravity; the frequency of light from the object appears to decrease, making it look redder, because the light appears to complete fewer cycles per “tick” of the observer’s clock; lower-frequency light has less energy and therefore appears dimmer. As an in falling object approaches the singularity, tidal forces acting on it approach infinity. All components of the object, including atoms and subatomic particles, are torn away from each other before striking the singularity.

How can we know Black Holes exists ?

Since black holes are small (only a few to a few tens of kilometers in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. We can’t see a black hole directly since light can’t get past the horizon. That means that we have to rely on indirect evidence that black holes exist.

Accretion disks and gas jets

Extremely large accretion disks and gas jets may be good evidence for the presence of super massive black holes, because as far as we know any mass large enough to power these phenomena must be a black hole.

Strong radiation emissions

Neutron stars and other very dense stars have surfaces, and matter colliding with the surface at a high percentage of the speed of light will produce intense flares of radiation at irregular intervals. Black holes have no material surface, so the absence of irregular flares round a massive, ultra-dense object suggests that there is a good chance of finding a black hole there.

Intense but one-time gamma ray bursts (GRBs) may signal the birth of “new” black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars or by collisions between neutron stars and both types of event involve sufficient mass and pressure to produce black holes. But it appears that a collision between a neutron star and a black hole can also cause a GRB, so a GRB is not proof that a “new” black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away so the black holes associated with them are actually billions of years old.

Gravitational lensing

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is “bent” around a massive object (such as a black hole) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein’s general theory of relativity. According to this theory, mass “warps” space-time to create gravitational fields and therefore bend light as a result.

Objects orbiting possible black holes

Many stars come in binary systems — pairs of stars in orbit around each other. If one of the stars in such a binary system becomes a black hole, we might be able to detect it. In particular, in some binary systems containing a compact object such as a black hole, matter is sucked off of the other object and forms an “accretion disk” of stuff swirling into the black hole. The matter in the accretion disk gets very hot as it falls closer and closer to the black hole, and it emits copious amounts of radiation, mostly in the X-ray part of the spectrum. Many such “X-ray binary systems” are known, and some of them are thought to be likely black-hole candidates.

Micro black holes

There is a theoretical possibility that a micro black hole might be created inside a particle accelerator. These black holes are not the same as gravitational black holes, but they are vital testing grounds for quantum theories of gravity.

Black Holes evaporation?

In 1970’s, Stephen Hawking came up with theoretical arguments showing that black holes are not really entirely black: due to quantum-mechanical effects, they emit radiation. The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

White Holes

The equations of general relativity have an interesting mathematical property: they are symmetric in time. That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you’ll get another valid solution to the equations. If you apply this rule to the solution that describes black holes, you get an object known as a white hole. Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out.

White holes are a perfectly valid mathematical solution to the equations of general relativity, but that doesn’t mean that they actually exist in nature. In fact, they almost certainly do not exist, since there’s no way to produce one. Producing a white hole is just as impossible as destroying a black hole.

Wormhole

We have been talking all along about black holes that are not rotating and have no electric charge. If we consider black holes that rotate and/or have charge, things get more complicated. In particular, it is possible to fall into such a black hole and not hit the singularity. In effect, the interior of a charged or rotating black hole can “join up” with a corresponding white hole in such a way that you can fall into the black hole and pop out of the white hole. This combination of black and white holes is called a wormhole.

Unfortunately, worm holes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat – the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them.

Black holes, Solar System and Earth

Black holes are sometimes listed among the most serious potential threats to Earth and humanity, on the grounds that a naturally-produced black hole could pass through our Solar System.

Stellar-mass black holes travel through the Milky Way just like stars. Consequently, they may collide with the Solar System or another planetary system in the galaxy, although the probability of this happening is very small. Significant gravitational interactions between the Sun and any other star in the Milky Way (including a black hole) are expected to occur approximately once every 10¹⁹ years. For comparison, the Sun has an age of only 5 × 10⁹ years, and is expected to become a red giant about 5 × 10⁹ years from now, incinerating the surface of the Earth. Hence it is extremely unlikely that a black hole will pass through the Solar System before the Sun exterminates life on Earth.

Random Walk

This random walk can be described statistically. We can estimate the total distance covered by the photon before final escape and at any time in its travel, we can predict the approximate distance of the photon from the star. An individual photon at the stellar core may be absorbed, re-emitted, absorbed again and re-emitted many times in succession. The direction in which photon is emitted may bear no relation at all to the direction in which it was traveling just before absorption. The photon may then travel randomly, until it reaches the surface of the star.

Radiative energy transport can be described as a random walk, where a photon repeatedly absorbed and re-emitted in a random direction. Let the step length of the walk (the mean free path) be d. Consider, for simplicity, the random walk in a plane. After one step the photon is absorbed at

where is an angle giving the direction of the step. After N steps the coordinates are

and the distance from the starting point is,

The first term in square brackets can be written as,

Since the directions are randomly distributed and independent,

The same result applies for the second term in square brackets. Then,

After N steps the photon is at the distance from the starting point. Given a mean free path of photon d and a random distribution of scattering angles, determine overall displacement r after a certain number N scattering events.

Thus, the statistical average of the overall displacement r after N steps is simply .

The time taken by photon to reach the surface from the center depends on the mean free path that is d.

We know that

Here, k is the mass absorption coefficient and is the density (that is mean density of the radiative core). For a typical 1 solar mass star,

Thus,

Now, the number of steps needed to reach radius r,

We know that the solar radius

Thus the number of steps is

The total path traveled by the photon is

and the time taken is

This is a very rough and highly approximated calculation and gives a time of as the time taken by the photon to reach the surface of the Sun.

The dark matter component has much more mass than the “visible” components of the universe. At present, density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stronger component, distributed diffusely in space. Some hard to detect Baryonic matter is believed to make a contribution of dark matter but would constitute a small portion. Determining the nature of this missing mass on one of the most important problems in modern cosmology and particle physics.

The vast majority of dark matter in the universe is believed to be non-baryonic, which means that it contains no atoms and it does not interact with ordinary matter via electromagnetic forces. The non-baryobnic dark matter includes neutrinos which were discovered to have mass in recent years and may also include hypothetical entities such as axions or super-symmetric particles.

]]> https://physicsanduniverse.com/dark-matter/feed/ 0 555 https://physicsanduniverse.com/galaxy-rotation-curve-dark-matter/ https://physicsanduniverse.com/galaxy-rotation-curve-dark-matter/#respond Fri, 21 Feb 2014 05:55:30 +0000 http://physicsanduniverse.com/?p=552 Stars revolve around the center of galaxies at a constant speed over a large range of distances from the center of the galaxy. It is observationally found that the stars revolve much faster than expected if they were in the free Newtonian potential. The rotation curve of a galaxy can be represented by a graph between orbital velocity of the stars or gas in the galaxy on the y axis and the distance from the center of the galaxy.

The galaxy rotation problem is the discrepancy between the observed rotation speeds of matter in the disc portions of the spiral galaxies and the predictions of Newtonian dynamics considering the known mass. This discrepancy is thought to be because of the presence of dark matter in the halo. The initial part of the curve represents the massive central part and this behavior is attributed to the presence of black hole.

Rotation curve of a typical spiral galaxy. Lower curve is a predicted motion and upper one is the observed. This discrepancy between the curves is attributed to dark matter.

Evidence of dark matter has been confirmed through the study of galaxy rotation curves. These measurements are on a smaller scale than the galaxy clusters, but gives more detail about the way the dark is distributed.

It is found that the stellar rotational velocity remains constant or “flat” with increasing distance away from the galactic center. This flat curve suggests that each galaxy is surrounded by significant amounts of dark matter. Unfortunately, because gravity depends only on the distance and mass; not on the composition, astronomer are not certain what the dark matter is composed of. It could be in the form of planets, brown dwarfs, white dwarfs, black holes, neutrinos with mass or other exotic particles that have not been discovered in the laboratory yet.

Since the Hubble constant is only a constant in space, not in time, the radius of the Hubble sphere may increase or decrease over various time intervals. The subscript ‘0’ in Hubble constant indicates the value of the Hubble constant today. Current evidence suggests that the expansion of the universe is accelerating. This means that for a given galaxy, the recession velocity is increasing over time as the galaxy moves to greater and greater distances. However Hubble parameter is actually thought to be decreasing with time. This means that if we were to look at some fixed distance D and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones.

The redshift is usually described as a ‘redshift velocity’, which is the recessional velocity that would produce the same redshift if it were caused by a linear Doppler effect.

here,

here, and are the observed and emitted wavelengths. The redshift velocity is not so simply related to real velocity at larger velocities.

The value of Hubble parameter changes over time either increasing or decreasing depending on the sign of the declaration parameter q which is defined by,

Here, q determine the nature of the space that is overall geometry of the universe. In a universe with a declaration parameter equal to zero (flat curvature), it follows that

by integration

So, we get

or,

the time t is the time since the Big Bang. A non zero, time dependent value of q simply requires integration of the Friedmann equations backwards from the present time when the co-moving horizon size was zero.

Thus, the age and ultimate fate of the universe can be determined by measuring the Hubble constant today and extrapolating with the observed value of the declaration parameter characterized by values of density parameters ( for matter and for dark energy).

It was long thought that q was positive indicating that the expansion is slowing down due to gravitational attraction. This would imply an age of the universe less than (which is about 14 billion years). For instance, a value of q is (once favored by theorists) would give the age of the universe as . In the year 1998, it was discovered that q is apparently negative, meaning that the universe could actually be older than . However the estimate regarding the age of the universe are very close to .

1. Star formation caused by density waves in the galactic disk of the galaxy
2. The SSPSF model: Star formation caused by shock waves in the interstellar medium

Density wave model: Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotates more slowly than the galaxy’s stars and gas. As gases enters a density wave, it gets squeezed and makes new stars. This idea was developed into density wave theory by Lin and Shu in 1964. They suggested that spiral arms were manifestations of spiral density waves. They assumed that stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated. In other words, the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center.

It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. The following hypothesis exists for star formation caused by density waves:

• As gas cloud moves into the density wave, the local mass density increases. Since the criteria for cloud collapse (Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
• As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
• As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.