Glossary

5.3 Star formation rate

8 min read•august 20, 2024

is a crucial metric for understanding galaxy evolution and cosmic history. It quantifies the mass of stars formed per unit time in a galaxy, typically expressed in solar masses per year.

Measuring star formation rates involves various techniques, each sensitive to different timescales and stellar populations. Factors influencing star formation include gas density, , and from and .

Star formation rate overview

  • is a crucial parameter in understanding galaxy evolution and the cosmic history of star formation
  • SFR quantifies the mass of stars formed per unit time in a galaxy or region, typically expressed in solar masses per year (M⊙/yrM_{\odot}/yr)
  • Studying SFR across different galaxy types, redshifts, and environments provides insights into the physical processes governing star formation and galaxy growth

Measuring star formation rates

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  • Various techniques are employed to measure SFR, each sensitive to different timescales and stellar populations
  • Common SFR indicators include ultraviolet continuum emission, recombination lines (Hα\alpha), infrared emission, and radio emission
  • Combining multiple SFR indicators helps mitigate biases and uncertainties associated with individual methods

Factors influencing star formation

  • Star formation is regulated by a complex interplay of physical processes, including gas density, turbulence, and feedback effects
  • Gas density plays a crucial role in star formation, with higher densities leading to increased SFR ()
  • Turbulence in the interstellar medium can both trigger and suppress star formation by creating overdensities and dispersing gas clouds
  • Feedback from stellar winds, supernovae, and radiation can heat and expel gas, regulating the efficiency of star formation

Molecular clouds and star formation

  • Star formation primarily occurs in dense, cold composed of molecular hydrogen (H2_2)
  • Molecular clouds undergo , fragmentation, and accretion to form protostars and ultimately main-sequence stars
  • The efficiency of star formation in molecular clouds is relatively low, with only a small fraction of the gas mass converted into stars

Star formation rate vs galaxy type

  • SFR varies significantly across different galaxy types, reflecting their distinct physical properties and evolutionary stages

Elliptical galaxies

  • Elliptical galaxies typically have low current SFRs, as they are dominated by older, redder stellar populations
  • Most of the star formation in elliptical galaxies occurred in the past, with their gas reservoir depleted or heated, suppressing further star formation

Spiral galaxies

  • Spiral galaxies exhibit a wide range of SFRs, with higher rates in gas-rich, late-type spirals compared to early-type spirals
  • Star formation in spiral galaxies is concentrated in the spiral arms, where gas density is higher and triggered by density waves
  • The SFR in spiral galaxies is sustained by ongoing gas accretion and recycling from stellar evolution

Irregular galaxies

  • Irregular galaxies often have high SFRs relative to their mass, due to their gas-rich nature and turbulent environments
  • , a subset of irregular galaxies, experience intense episodes of star formation triggered by galaxy interactions or mergers
  • The high SFRs in irregular galaxies can be short-lived, as the available gas is rapidly consumed or expelled by feedback processes

Star formation rate vs redshift

  • The cosmic star formation history describes the evolution of the global SFR density over cosmic time, peaking at redshifts z≈1−3z \approx 1-3

Low redshift galaxies

  • In the local universe (z<0.5z < 0.5), SFRs are generally lower compared to the cosmic peak, with a mix of quiescent and star-forming galaxies
  • Low redshift star-forming galaxies are typically less massive, less luminous, and have lower gas fractions than their high-redshift counterparts

High redshift galaxies

  • At high redshifts (z>1z > 1), SFRs are significantly higher, with a larger fraction of galaxies experiencing intense star formation
  • High-redshift galaxies are more gas-rich, compact, and turbulent, leading to enhanced star formation activity
  • The peak of cosmic star formation at z≈2z \approx 2 corresponds to the era of maximum galaxy growth and assembly

Cosmic star formation history

  • The cosmic star formation history traces the evolution of the global SFR density over cosmic time
  • It shows a rapid rise from early times, peaking at z≈2z \approx 2, followed by a decline towards the present day
  • The shape of the cosmic star formation history is shaped by the interplay of gas accretion, feedback processes, and the evolving galaxy population

Star formation rate indicators

  • Different observational tracers are used to estimate SFRs, each with its own strengths, limitations, and assumptions

Ultraviolet continuum

  • UV continuum emission (1250-2500 Å) directly traces the photospheric emission from young, massive stars (O and B stars)
  • UV-based SFRs probe recent star formation on timescales of ∼\sim100 Myr, but are sensitive to dust extinction
  • Dust corrections are necessary to account for the attenuation of UV light, especially in dusty, star-forming galaxies

Recombination lines

  • Recombination lines, such as Hα\alpha (6563 Å), arise from the ionized gas surrounding massive stars in H II regions
  • Hα\alpha emission provides a nearly instantaneous measure of the SFR, tracing stars with ages <10<10 Myr
  • Recombination lines are less affected by dust than UV emission but still require extinction corrections

Infrared emission

  • Dust heated by young, massive stars emits thermal radiation in the infrared (IR) wavelengths (8-1000 μ\mum)
  • IR emission traces dust-obscured star formation, capturing the SFR missed by UV and optical indicators
  • Total IR luminosity is often used as an SFR indicator, assuming a constant dust heating efficiency and initial mass function

Radio emission

  • Radio continuum emission at 1.4 GHz is a tracer of SFR, arising from synchrotron radiation from relativistic electrons and free-free emission from H II regions
  • Radio SFR indicators are unaffected by dust attenuation, making them valuable for dusty, star-forming galaxies
  • The radio-SFR relation is based on the tight correlation between radio and far-infrared emission, known as the radio-FIR correlation

Star formation laws and models

  • Star formation laws and models aim to describe the relationship between gas density and SFR, providing insights into the physical processes regulating star formation

Schmidt-Kennicutt law

  • The Schmidt-Kennicutt (SK) law is an empirical relation between the surface density of gas (Σgas\Sigma_{gas}) and the surface density of star formation (ΣSFR\Sigma_{SFR})
  • The SK law is typically expressed as a power law: ΣSFR∝ΣgasN\Sigma_{SFR} \propto \Sigma_{gas}^N, with N≈1.4N \approx 1.4
  • The SK law holds on global scales for disk galaxies, but breaks down on smaller scales and in extreme environments

Gas density and star formation

  • Gas density is a key factor in regulating star formation, as higher densities lead to increased gravitational instability and collapse
  • The critical gas density for star formation depends on the balance between gravity and turbulent support
  • Molecular gas (H2_2) is more directly related to star formation than atomic hydrogen (H I), as it traces the dense, star-forming regions

Turbulence and star formation

  • Turbulence in the interstellar medium plays a dual role in star formation, both triggering and suppressing it
  • Turbulence can create local overdensities and compress gas, leading to gravitational collapse and star formation
  • However, turbulence can also provide support against collapse, increasing the critical gas density for star formation

Feedback effects on star formation

  • Stellar feedback processes, such as stellar winds, supernovae, and radiation, can significantly impact star formation
  • Feedback can heat and expel gas from star-forming regions, reducing the efficiency of star formation and regulating the global SFR
  • Positive feedback can also occur, where compression from shocks and turbulence triggered by feedback leads to enhanced star formation

Environmental effects on star formation

  • The environment in which galaxies reside can strongly influence their star formation properties

Galaxy mergers and interactions

  • Galaxy mergers and interactions can trigger intense starbursts by funneling gas into the central regions and inducing gravitational instabilities
  • The enhanced SFRs in merging systems can be an order of magnitude higher than in isolated galaxies
  • The merger-induced star formation depends on the mass ratio, gas content, and orbital parameters of the interacting galaxies

Tidal effects and star formation

  • Tidal interactions between galaxies can lead to the formation of tidal tails, bridges, and shells, which can host star-forming regions
  • Tidal compression and shocks can trigger star formation in the outskirts of interacting galaxies
  • However, tidal stripping can also remove gas from galaxies, reducing their potential for future star formation

Starburst galaxies and star formation

  • Starburst galaxies experience exceptionally high SFRs, often triggered by mergers or interactions
  • The intense star formation in starbursts can deplete the gas reservoir on short timescales (<100<100 Myr), leading to a rapid decline in SFR
  • Feedback from the starburst can drive powerful outflows, expelling gas and regulating the star formation efficiency

Stellar initial mass function

  • The stellar describes the distribution of initial masses for a population of stars formed in a single star formation event

IMF variations across galaxies

  • The shape of the IMF can vary across different galaxies and star-forming environments
  • Some studies suggest a top-heavy IMF in starburst galaxies, with a higher fraction of massive stars compared to the Milky Way IMF
  • Variations in the IMF can impact the derived SFRs, as different indicators trace different stellar mass ranges

IMF impact on star formation rates

  • The choice of IMF affects the conversion factors used to derive SFRs from observational tracers
  • A top-heavy IMF implies a higher proportion of massive stars, leading to higher luminosities and ionizing photon rates per unit mass of stars formed
  • Uncertainties in the IMF can introduce systematic uncertainties in the derived SFRs and the cosmic star formation history

Star formation and galaxy evolution

  • Star formation plays a crucial role in the evolution of galaxies, shaping their properties and chemical composition

Stellar population synthesis models

  • are used to interpret the observed properties of galaxies in terms of their star formation histories
  • These models combine stellar evolutionary tracks, stellar atmospheres, and an assumed IMF to predict the spectral energy distribution of a galaxy
  • By fitting observed galaxy spectra with these models, the star formation history and stellar populations of galaxies can be inferred

Chemical enrichment from star formation

  • Star formation drives the chemical evolution of galaxies through the production of heavy elements in stellar nucleosynthesis
  • Massive stars (>8 M⊙M_{\odot}) contribute to the rapid enrichment of the interstellar medium through their stellar winds and supernova explosions
  • The chemical abundance patterns in galaxies provide insights into their star formation histories and the efficiency of galactic outflows

Star formation and galaxy mass assembly

  • Star formation is a key process in the growth and assembly of galaxies over cosmic time
  • The stellar mass of a galaxy is built up through a combination of in-situ star formation and galaxy mergers
  • The relative importance of these two channels depends on the galaxy mass, redshift, and environment
  • Feedback from star formation can regulate the gas supply and limit the maximum stellar mass of galaxies

Key Terms to Review (22)

Chemical enrichment: Chemical enrichment refers to the process by which heavier elements are produced and distributed throughout the universe, primarily through the lifecycle of stars. As stars form, evolve, and eventually explode as supernovae, they release these newly synthesized elements into the surrounding interstellar medium, enhancing the chemical composition of future generations of stars and planets. This process plays a critical role in determining star formation rates, influences the initial mass function of new stars, and is significantly affected by stellar feedback mechanisms.
Cloud fragmentation theory: Cloud fragmentation theory explains the process by which molecular clouds, the dense regions of gas and dust in space, break apart into smaller clumps that can collapse under their own gravity to form stars. This theory is crucial for understanding how the star formation rate varies across different environments in galaxies, as it emphasizes the importance of cloud dynamics and initial conditions in determining the number of stars formed over time.
Competitive accretion theory: Competitive accretion theory is a concept in astrophysics that explains how stars form in dense molecular clouds by suggesting that multiple protostars compete for the available gas and dust. This competition drives the rate of star formation, as more massive stars grow faster by attracting more material than their less massive counterparts, leading to a hierarchical structure of star formation where mass plays a critical role.
Feedback effects: Feedback effects refer to the processes in which the output of a system influences its own input, often creating a loop of interaction that can enhance or dampen specific outcomes. In the context of star formation and HII regions, feedback effects play a crucial role in regulating the environment, influencing the rate of star formation, and determining the lifecycle of stars and the surrounding gas.
Galactic mergers: Galactic mergers refer to the process where two or more galaxies collide and combine to form a single, larger galaxy. This phenomenon significantly influences the evolution of galaxies, impacting their structure, dynamics, and star formation rates. When galaxies merge, they trigger gravitational interactions that can lead to bursts of star formation and the formation of new stars from the gas and dust within the galaxies.
Gravitational Collapse: Gravitational collapse is the process by which an astronomical object contracts under its own gravity, leading to the formation of denser structures like stars, galaxies, or even black holes. This fundamental process plays a critical role in the evolution of the universe, shaping the distribution of matter and energy throughout space and influencing the formation of cosmic structures.
Initial Mass Function (IMF): The Initial Mass Function (IMF) describes the distribution of masses for a population of stars at the time of their formation. It is crucial for understanding star formation rates and how different stellar masses contribute to the evolution of galaxies. The IMF provides insights into the relationship between mass and luminosity, helping astronomers determine the total stellar mass in a galaxy and assess how star formation influences galactic dynamics over time.
Jules Henri Poincaré: Jules Henri Poincaré was a French mathematician, physicist, and philosopher of science, recognized for his foundational contributions to topology, celestial mechanics, and the theory of dynamical systems. His work laid the groundwork for understanding complex systems and their behaviors, which is crucial in studying the star formation rate as it involves analyzing various dynamic processes in astrophysics.
Main sequence: The main sequence is a continuous and distinctive band of stars that appears on a Hertzsprung-Russell diagram, where stars spend the majority of their lifetimes fusing hydrogen into helium in their cores. This phase is crucial in the life cycle of stars, as it reflects the balance between gravitational collapse and the outward pressure generated by nuclear fusion. Understanding the main sequence helps astronomers determine a star's mass, age, and evolutionary path.
Molecular clouds: Molecular clouds are dense regions of gas and dust in space, primarily composed of hydrogen molecules. They serve as the primary sites for star formation, providing the necessary conditions for gravity to collapse and form stars. These clouds are typically cold, with temperatures around 10-20 K, and their high density allows them to shield molecules from dissociation by ultraviolet radiation, making them crucial for the formation of new stars and planetary systems.
Population I stars: Population I stars are a category of stars that are relatively young, metal-rich, and typically found in the disk of galaxies, including our Milky Way. These stars are associated with ongoing star formation and play a crucial role in understanding the chemical evolution of galaxies and the dynamics of star formation rates.
Population II stars: Population II stars are a class of stars that are older, metal-poor, and typically found in the halo and globular clusters of galaxies. They play a crucial role in understanding the early universe, as they formed from primordial gas before significant amounts of heavy elements were produced by earlier generations of stars. Their existence provides insights into the processes of galaxy formation and the history of star formation across cosmic time.
Protostar: A protostar is an early stage in the formation of a star, occurring when a cloud of gas and dust collapses under its own gravity, leading to the accumulation of material in a dense core. This process eventually leads to the ignition of nuclear fusion, marking the transition from a protostar to a main-sequence star. During this phase, the protostar undergoes significant changes and is still gathering mass from its surrounding environment.
Schmidt-Kennicutt Law: The Schmidt-Kennicutt Law describes the relationship between the star formation rate (SFR) and the gas surface density in a galaxy. This empirical law indicates that regions with higher gas densities tend to form stars at a higher rate, demonstrating how efficiently galaxies convert gas into new stars, which is crucial for understanding galaxy evolution.
Star formation rate: Star formation rate (SFR) is a measure of the amount of mass converted into stars in a specific region of space over a given time, typically expressed in solar masses per year. This concept is crucial in understanding the evolution of galaxies, as it directly relates to their structure, characteristics, and lifecycle, affecting various classifications and types of galaxies, especially spiral galaxies where star formation is often actively occurring.
Star Formation Rate (SFR): Star Formation Rate (SFR) is a measure of the rate at which new stars are formed in a given volume of space, typically expressed in solar masses per year. This rate is crucial for understanding the evolution of galaxies, as it indicates how effectively a galaxy is creating new stars, which impacts its overall mass, luminosity, and life cycle. High SFR values suggest that a galaxy is in a period of vigorous star formation, often influenced by factors like gas density and environmental conditions.
Starburst galaxies: Starburst galaxies are a class of galaxies that experience an exceptionally high rate of star formation, often several times greater than that of typical galaxies. This intense star formation is usually triggered by interactions such as mergers with other galaxies or gravitational forces, leading to a burst of activity within the galaxy. The properties and phenomena associated with starburst galaxies connect to various aspects like HII regions, star formation rates, and hierarchical merging.
Stellar population synthesis models: Stellar population synthesis models are theoretical frameworks used to understand and simulate the light emitted by a group of stars, taking into account their different ages, compositions, and evolutionary stages. These models help astronomers analyze the star formation history and current star formation rate in galaxies by combining stellar evolution tracks with initial mass functions. By predicting how light varies across different wavelengths, they provide insights into the characteristics and dynamics of star clusters and galaxies over time.
Stellar winds: Stellar winds are streams of charged particles, primarily electrons and protons, that are ejected from the outer layers of a star into space. These winds play a crucial role in shaping the environment around stars, influencing the formation of new stars and the evolution of galaxies. They also contribute to processes such as chemical enrichment of the interstellar medium and feedback mechanisms in star formation.
Subrahmanyan Chandrasekhar: Subrahmanyan Chandrasekhar was an Indian-American astrophysicist who made significant contributions to the understanding of stellar evolution and the structure of stars. His most notable achievement is the formulation of the Chandrasekhar limit, which defines the maximum mass of a stable white dwarf star, thus influencing theories about star formation rates and the fate of massive stars.
Supernovae: Supernovae are powerful and luminous explosions that occur at the end of a star's life cycle, marking the transition from a stable phase to a spectacular event. These explosions can significantly influence their surrounding environment, enriching the interstellar medium with heavy elements and impacting star formation in nearby regions. They also serve as critical distance indicators in the universe and play a role in understanding cosmic expansion.
Turbulence: Turbulence refers to the chaotic and irregular motion of fluids, which can significantly impact the dynamics of astrophysical environments. In space, turbulence plays a critical role in the behavior of accretion disks around celestial objects, influences the structure and evolution of molecular clouds, and affects the star formation rate by mixing gas and dust, leading to localized regions of collapse. Understanding turbulence is key to grasping how these cosmic structures evolve and interact.
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